import java.io.InputStream;
import java.util.Arrays;
import java.util.NoSuchElementException;
import java.util.OptionalInt;
import java.util.PrimitiveIterator;
import java.util.function.IntUnaryOperator;
import java.util.function.LongUnaryOperator;


public class Main {
    public static void main(String[] args) throws Exception {
        ExtendedScanner sc = new ExtendedScanner();
        FastPrintStream pw = new FastPrintStream();
        solve(sc, pw);
        sc.close();
        pw.flush();
        pw.close();
    }

    public static void solve(ExtendedScanner sc, FastPrintStream pw) {
        int n = sc.nextInt();
        int m = sc.nextInt();
        int a = sc.nextInt() - 1;
        int b = sc.nextInt();
        Graph<SimpleEdge> g = new Graph<>(n + 1);
        for (int i = 0; i < m; i++) {
            int l = sc.nextInt() - 1;
            int r = sc.nextInt();
            g.addEdge(new SimpleEdge(l, r));
        }
        int[] d = new int[n + 1];
        Arrays.fill(d, a + 1, n + 1, -1);
        IntDeque dq = new IntDeque();
        for (int i = 0; i <= a; i++) {
            dq.addLast(i);
        }
        while (dq.size() > 0) {
            int u = dq.removeFirst();
            for (var e : g.getEdges(u)) {
                int v = e.to;
                if (v >= b) {
                    pw.println(d[u] + 1);
                    return;
                } else if (d[v] < 0) {
                    d[v] = d[u] + 1;
                    dq.addLast(v);
                }
            }
        }
        pw.println(-1);
    }
}

/**
 * @author https://atcoder.jp/users/suisen
 */
class FastScanner implements AutoCloseable {
    private final ByteBuffer tokenBuf = new ByteBuffer();
    private final java.io.InputStream in;
    private final byte[] rawBuf = new byte[1 << 14];
    private int ptr = 0;
    private int buflen = 0;

    public FastScanner(java.io.InputStream in) {
        this.in = in;
    }

    public FastScanner() {
        this(new java.io.FileInputStream(java.io.FileDescriptor.in));
    }

    private final int readByte() {
        if (ptr < buflen) return rawBuf[ptr++];
        ptr = 0;
        try {
            buflen = in.read(rawBuf);
            if (buflen > 0) {
                return rawBuf[ptr++];
            } else {
                throw new java.io.EOFException();
            }
        } catch (java.io.IOException e) {
            throw new java.io.UncheckedIOException(e);
        }
    }

    private final int readByteUnsafe() {
        if (ptr < buflen) return rawBuf[ptr++];
        ptr = 0;
        try {
            buflen = in.read(rawBuf);
            if (buflen > 0) {
                return rawBuf[ptr++];
            } else {
                return -1;
            }
        } catch (java.io.IOException e) {
            throw new java.io.UncheckedIOException(e);
        }
    }

    private final int skipUnprintableChars() {
        int b = readByte();
        while (b <= 32 || b >= 127) b = readByte();
        return b;
    }

    private final void loadToken() {
        tokenBuf.clear();
        for (int b = skipUnprintableChars(); 32 < b && b < 127; b = readByteUnsafe()) {
            tokenBuf.append(b);
        }
    }

    public final boolean hasNext() {
        for (int b = readByteUnsafe(); b <= 32 || b >= 127; b = readByteUnsafe()) {
            if (b == -1) return false;
        }
        --ptr;
        return true;
    }

    public final String next() {
        loadToken();
        return new String(tokenBuf.getRawBuf(), 0, tokenBuf.size());
    }

    public final String nextLine() {
        tokenBuf.clear();
        for (int b = readByte(); b != '\n'; b = readByteUnsafe()) {
            if (b == -1) break;
            tokenBuf.append(b);
        }
        return new String(tokenBuf.getRawBuf(), 0, tokenBuf.size());
    }

    public final char nextChar() {
        return (char) skipUnprintableChars();
    }

    public final char[] nextChars() {
        loadToken();
        return tokenBuf.toCharArray();
    }

    public final long nextLong() {
        long n = 0;
        boolean isNegative = false;
        int b = skipUnprintableChars();
        if (b == '-') {
            isNegative = true;
            b = readByteUnsafe();
        }
        if (b < '0' || '9' < b) throw new NumberFormatException();
        while ('0' <= b && b <= '9') {
            // -9223372036854775808 - 9223372036854775807
            if (n >= 922337203685477580l) {
                if (n > 922337203685477580l) {
                    throw new ArithmeticException("long overflow");
                }
                if (isNegative) {
                    if (b >= '9') {
                        throw new ArithmeticException("long overflow");
                    }
                    n = -n - (b + '0');
                    b = readByteUnsafe();
                    if ('0' <= b && b <= '9') {
                        throw new ArithmeticException("long overflow");
                    } else if (b <= 32 || b >= 127) {
                        return n;
                    } else {
                        throw new NumberFormatException();
                    }
                } else {
                    if (b >= '8') {
                        throw new ArithmeticException("long overflow");
                    }
                    n = n * 10 + b - '0';
                    b = readByteUnsafe();
                    if ('0' <= b && b <= '9') {
                        throw new ArithmeticException("long overflow");
                    } else if (b <= 32 || b >= 127) {
                        return n;
                    } else {
                        throw new NumberFormatException();
                    }
                }
            }
            n = n * 10 + b - '0';
            b = readByteUnsafe();
        }
        if (b <= 32 || b >= 127) return isNegative ? -n : n;
        throw new NumberFormatException();
    }
    public final int nextInt() {
        long value = nextLong();
        if ((int) value != value) {
            throw new ArithmeticException("int overflow");
        }
        return (int) value;
    }
    public final double nextDouble() {
        return Double.parseDouble(next());
    }
    public final void close() {
        try {
            in.close();
        } catch (java.io.IOException e) {
            throw new java.io.UncheckedIOException(e);
        }
    }

    private static final class ByteBuffer {
        private static final int DEFAULT_BUF_SIZE = 1 << 12;
        private byte[] buf;
        private int ptr = 0;
        private ByteBuffer(int capacity) {
            this.buf = new byte[capacity];
        }
        private ByteBuffer() {
            this(DEFAULT_BUF_SIZE);
        }
        private ByteBuffer append(int b) {
            if (ptr == buf.length) {
                int newLength = buf.length << 1;
                byte[] newBuf = new byte[newLength];
                System.arraycopy(buf, 0, newBuf, 0, buf.length);
                buf = newBuf;
            }
            buf[ptr++] = (byte) b;
            return this;
        }
        private char[] toCharArray() {
            char[] chs = new char[ptr];
            for (int i = 0; i < ptr; i++) {
                chs[i] = (char) buf[i];
            }
            return chs;
        }
        private byte[] getRawBuf() {
            return buf;
        }
        private int size() {
            return ptr;
        }
        private void clear() {
            ptr = 0;
        }
    }
}


/**
 * @author https://atcoder.jp/users/suisen
 */
final class ExtendedScanner extends FastScanner {
    public ExtendedScanner() {super();}
    public ExtendedScanner(InputStream in) {super(in);}
    public int[] ints(final int n) {
        final int[] a = new int[n];
        Arrays.setAll(a, $ -> nextInt());
        return a;
    }
    public int[] ints(final int n, final IntUnaryOperator f) {
        final int[] a = new int[n];
        Arrays.setAll(a, $ -> f.applyAsInt(nextInt()));
        return a;
    }
    public int[][] ints(final int n, final int m) {
        final int[][] a = new int[n][];
        Arrays.setAll(a, $ -> ints(m));
        return a;
    }
    public int[][] ints(final int n, final int m, final IntUnaryOperator f) {
        final int[][] a = new int[n][];
        Arrays.setAll(a, $ -> ints(m, f));
        return a;
    }
    public long[] longs(final int n) {
        final long[] a = new long[n];
        Arrays.setAll(a, $ -> nextLong());
        return a;
    }
    public long[] longs(final int n, final LongUnaryOperator f) {
        final long[] a = new long[n];
        Arrays.setAll(a, $ -> f.applyAsLong(nextLong()));
        return a;
    }
    public long[][] longs(final int n, final int m) {
        final long[][] a = new long[n][];
        Arrays.setAll(a, $ -> longs(m));
        return a;
    }
    public long[][] longs(final int n, final int m, final LongUnaryOperator f) {
        final long[][] a = new long[n][];
        Arrays.setAll(a, $ -> longs(m, f));
        return a;
    }
    public char[][] charArrays(final int n) {
        final char[][] c = new char[n][];
        Arrays.setAll(c, $ -> nextChars());
        return c;
    }
    public double[] doubles(final int n) {
        final double[] a = new double[n];
        Arrays.setAll(a, $ -> nextDouble());
        return a;
    }
    public double[][] doubles(final int n, final int m) {
        final double[][] a = new double[n][];
        Arrays.setAll(a, $ -> doubles(m));
        return a;
    }
    public String[] strings(final int n) {
        final String[] s = new String[n];
        Arrays.setAll(s, $ -> next());
        return s;
    }
}

abstract class AbstractGraph<Edg extends AbstractEdge> {
    final int n;
    final java.util.ArrayList<Edg> edges;
    final java.util.ArrayList<java.util.ArrayList<Edg>> adj;
    public AbstractGraph(int n) {
        this.n = n;
        this.edges = new java.util.ArrayList<>(n);
        this.adj = new java.util.ArrayList<>(n);
        for (int i = 0; i < n; i++) {
            adj.add(new java.util.ArrayList<>());
        }
    }
    public abstract void addEdge(Edg edge);
    public Edg getEdge(int u, int i) {
        return adj.get(u).get(i);
    }
    public java.util.ArrayList<Edg> getEdges(int u) {
        return adj.get(u);
    }
    public java.util.ArrayList<Edg> getEdges() {
        return edges;
    }
    public int deg(int u) {
        return adj.get(u).size();
    }
    public int getV() {
        return n;
    }
    public int getE() {
        return edges.size();
    }
}

/**
 * @author https://atcoder.jp/users/suisen
 */
class FastPrintStream implements AutoCloseable {
    private static final int INT_MAX_LEN = 11;
    private static final int LONG_MAX_LEN = 20;

    private int precision = 9;

    private static final int BUF_SIZE = 1 << 14;
    private static final int BUF_SIZE_MINUS_INT_MAX_LEN = BUF_SIZE - INT_MAX_LEN;
    private static final int BUF_SIZE_MINUS_LONG_MAX_LEN = BUF_SIZE - LONG_MAX_LEN;
    private final byte[] buf = new byte[BUF_SIZE];
    private int ptr = 0;
    private final java.lang.reflect.Field strField;
    private final java.nio.charset.CharsetEncoder encoder;

    private final java.io.OutputStream out;

    public FastPrintStream(java.io.OutputStream out) {
        this.out = out;
        java.lang.reflect.Field f;
        try {
            f = java.lang.String.class.getDeclaredField("value");
            f.setAccessible(true);
        } catch (NoSuchFieldException | SecurityException e) {
            f = null;
        }
        this.strField = f;
        this.encoder = java.nio.charset.StandardCharsets.US_ASCII.newEncoder();
    }

    public FastPrintStream(java.io.File file) throws java.io.IOException {
        this(new java.io.FileOutputStream(file));
    }

    public FastPrintStream(java.lang.String filename) throws java.io.IOException {
        this(new java.io.File(filename));
    }

    public FastPrintStream() {
        this(new java.io.FileOutputStream(java.io.FileDescriptor.out));
    }

    public FastPrintStream println() {
        if (ptr == BUF_SIZE) internalFlush();
        buf[ptr++] = (byte) '\n';
        return this;
    }

    public FastPrintStream println(java.lang.Object o) {
        return print(o).println();
    }

    public FastPrintStream println(java.lang.String s) {
        return print(s).println();
    }

    public FastPrintStream println(char[] s) {
        return print(s).println();
    }

    public FastPrintStream println(char c) {
        return print(c).println();
    }

    public FastPrintStream println(int x) {
        return print(x).println();
    }

    public FastPrintStream println(long x) {
        return print(x).println();
    }

    public FastPrintStream println(double d, int precision) {
        return print(d, precision).println();
    }

    public FastPrintStream println(double d) {
        return print(d).println();
    }

    private FastPrintStream print(byte[] bytes) {
        int n = bytes.length;
        if (ptr + n > BUF_SIZE) {
            internalFlush();
            try {
                out.write(bytes);
            } catch (java.io.IOException e) {
                throw new java.io.UncheckedIOException(e);
            }
        } else {
            System.arraycopy(bytes, 0, buf, ptr, n);
            ptr += n;
        }
        return this;
    }

    public FastPrintStream print(java.lang.Object o) {
        return print(o.toString());
    }

    public FastPrintStream print(java.lang.String s) {
        if (strField == null) {
            return print(s.getBytes());
        } else {
            try {
                Object value = strField.get(s);
                if (value instanceof byte[]) {
                    return print((byte[]) value);
                } else {
                    return print((char[]) value);
                }
            } catch (IllegalAccessException e) {
                return print(s.getBytes());
            }
        }
    }

    public FastPrintStream print(char[] s) {
        try {
            return print(encoder.encode(java.nio.CharBuffer.wrap(s)).array());
        } catch (java.nio.charset.CharacterCodingException e) {
            byte[] bytes = new byte[s.length];
            for (int i = 0; i < s.length; i++) {
                bytes[i] = (byte) s[i];
            }
            return print(bytes);
        }
    }

    public FastPrintStream print(char c) {
        if (ptr == BUF_SIZE) internalFlush();
        buf[ptr++] = (byte) c;
        return this;
    }

    public FastPrintStream print(int x) {
        if (ptr > BUF_SIZE_MINUS_INT_MAX_LEN) internalFlush();
        if (-10 < x && x < 10) {
            if (x < 0) {
                buf[ptr++] = '-';
                x = -x;
            }
            buf[ptr++] = (byte) ('0' + x);
            return this;
        }
        int d;
        if (x < 0) {
            if (x == Integer.MIN_VALUE) {
                buf[ptr++] = '-'; buf[ptr++] = '2'; buf[ptr++] = '1'; buf[ptr++] = '4';
                buf[ptr++] = '7'; buf[ptr++] = '4'; buf[ptr++] = '8'; buf[ptr++] = '3';
                buf[ptr++] = '6'; buf[ptr++] = '4'; buf[ptr++] = '8';
                return this;
            }
            d = len(x = -x);
            buf[ptr++] = '-';
        } else {
            d = len(x);
        }
        int j = ptr += d; 
        while (x > 0) {
            buf[--j] = (byte) ('0' + (x % 10));
            x /= 10;
        }
        return this;
    }

    public FastPrintStream print(long x) {
        if ((int) x == x) return print((int) x);
        if (ptr > BUF_SIZE_MINUS_LONG_MAX_LEN) internalFlush();
        int d;
        if (x < 0) {
            if (x == Long.MIN_VALUE) {
                buf[ptr++] = '-'; buf[ptr++] = '9'; buf[ptr++] = '2'; buf[ptr++] = '2';
                buf[ptr++] = '3'; buf[ptr++] = '3'; buf[ptr++] = '7'; buf[ptr++] = '2';
                buf[ptr++] = '0'; buf[ptr++] = '3'; buf[ptr++] = '6'; buf[ptr++] = '8';
                buf[ptr++] = '5'; buf[ptr++] = '4'; buf[ptr++] = '7'; buf[ptr++] = '7';
                buf[ptr++] = '5'; buf[ptr++] = '8'; buf[ptr++] = '0'; buf[ptr++] = '8';
                return this;
            }
            d = len(x = -x);
            buf[ptr++] = '-';
        } else {
            d = len(x);
        }
        int j = ptr += d; 
        while (x > 0) {
            buf[--j] = (byte) ('0' + (x % 10));
            x /= 10;
        }
        return this;
    }

    public FastPrintStream print(double d, int precision) {
        if (d < 0) {
            print('-');
            d = -d;
        }
        d += Math.pow(10, -precision) / 2;
        print((long) d).print('.');
        d -= (long) d;
        for(int i = 0; i < precision; i++){
            d *= 10;
            print((int) d);
            d -= (int) d;
        }
        return this;
    }

    public FastPrintStream print(double d) {
        return print(d, precision);
    }

    public void setPrecision(int precision) {
        this.precision = precision;
    }

    private void internalFlush() {
        try {
            out.write(buf, 0, ptr);
            ptr = 0;
        } catch (java.io.IOException e) {
            throw new java.io.UncheckedIOException(e);
        }
    }

    public void flush() {
        try {
            out.write(buf, 0, ptr);
            out.flush();
            ptr = 0;
        } catch (java.io.IOException e) {
            throw new java.io.UncheckedIOException(e);
        }
    }

    public void close() {
        try {
            out.close();
        } catch (java.io.IOException e) {
            throw new java.io.UncheckedIOException(e);
        }
    }

    private static int len(int x) {
        return
            x >= 1000000000 ? 10 :
            x >= 100000000  ?  9 :
            x >= 10000000   ?  8 :
            x >= 1000000    ?  7 :
            x >= 100000     ?  6 :
            x >= 10000      ?  5 :
            x >= 1000       ?  4 :
            x >= 100        ?  3 :
            x >= 10         ?  2 : 1;
    }

    private static int len(long x) {
        return
            x >= 1000000000000000000l ? 19 :
            x >= 100000000000000000l  ? 18 :
            x >= 10000000000000000l   ? 17 :
            x >= 1000000000000000l    ? 16 :
            x >= 100000000000000l     ? 15 :
            x >= 10000000000000l      ? 14 :
            x >= 1000000000000l       ? 13 :
            x >= 100000000000l        ? 12 :
            x >= 10000000000l         ? 11 : 10;
    }
}

final class SimpleEdge extends AbstractEdge {
    public SimpleEdge(int from, int to, long cost) {
        super(from, to, cost);
    }
    public SimpleEdge(int from, int to) {
        super(from, to);
    }
    @Override
    public SimpleEdge reverse() {
        return new SimpleEdge(to, from, cost);
    }
}


/**
 * @author https://atcoder.jp/users/suisen
 * 
 * Implementation of deque for primitive int type, using Ring Buffer.
 */
final class IntDeque implements Iterable<Integer> {
    static final int DEFAULT_CAPACITY = 1 << 6;
    int[] buf;
    int len = 1;
    int mask;
    int head = 0;
    int tail = 0;
    public IntDeque(int capacity) {
        if (capacity <= 0) {
            throw new IllegalArgumentException(
                String.format("Capacity %d is negative.", capacity)
            );
        }
        while (this.len < capacity) {
            this.len <<= 1;
        }
        this.mask = this.len - 1;
        this.buf = new int[len];
    }
    public IntDeque() {
        this(DEFAULT_CAPACITY);
    }
    public int getLast() {
        if (size() == 0) throw new NoSuchElementException();
        return buf[(tail - 1) & mask];
    }
    public int getFirst() {
        if (size() == 0) throw new NoSuchElementException();
        return buf[head & mask];
    }
    public int get(int index) {
        if (index < 0 || index >= size()) {
            throw new IndexOutOfBoundsException(
                String.format("Index %d out of bounds for length %d.", index, size())
            );
        }
        return buf[(head + index) & mask];
    }
    public void addLast(int v) {
        if (size() == len) grow();
        buf[tail++ & mask] = v;
    }
    public void addFirst(int v) {
        if (size() == len) grow();
        buf[--head & mask] = v;
    }
    public int removeLast() {
        if (size() == 0) throw new NoSuchElementException();
        return buf[--tail & mask];
    }
    public int removeFirst() {
        if (size() == 0) throw new NoSuchElementException();
        return buf[head++ & mask];
    }
    public OptionalInt pollLast() {
        if (size() == 0) return OptionalInt.empty();
        return OptionalInt.of(removeLast());
    }
    public OptionalInt pollFirst() {
        if (size() == 0) return OptionalInt.empty();
        return OptionalInt.of(removeFirst());
    }
    public int size() {
        return tail - head;
    }
    public boolean isEmpty() {
        return size() == 0;
    }
    public void clear() {
        head = tail = 0;
    }
    public int[] toArray() {
        int[] ret = new int[size()];
        PrimitiveIterator.OfInt it = iterator();
        Arrays.setAll(ret, i -> it.nextInt());
        return ret;
    }
    private void grow() {
        int[] newBuf = new int[len << 1];
        head &= mask;
        tail &= mask;
        int len1 = len - head;
        int len2 = head;
        System.arraycopy(buf, head, newBuf, 0, len1);
        System.arraycopy(buf, 0, newBuf, len1, len2);
        this.head = 0;
        this.tail = this.len;
        this.len <<= 1;
        this.mask = this.len - 1;
        this.buf = newBuf;
    }
    public PrimitiveIterator.OfInt iterator() {
        return new PrimitiveIterator.OfInt(){
            int it = head;
            public boolean hasNext() {return it < tail;}
            public int nextInt() {return buf[it++ & mask];}
        };
    }
    public PrimitiveIterator.OfInt descendingIterator() {
        return new PrimitiveIterator.OfInt(){
            int it = tail;
            public boolean hasNext() {return it > head;}
            public int nextInt() {return buf[--it & mask];}
        };
    }

    /***************************** DEBUG *********************************/

    @Override
    public String toString() {
        StringBuilder sb = new StringBuilder();
        sb.append('[');
        PrimitiveIterator.OfInt it = iterator();
        while (it.hasNext()) {
            sb.append(it.next().toString());
            if (it.hasNext()) sb.append(',');
        }
        sb.append(']');
        return sb.toString();
    }
}

abstract class AbstractEdge implements Comparable<AbstractEdge> {
    public final int from, to;
    public final long cost;
    public AbstractEdge(int from, int to, long cost) {
        this.from = from;
        this.to = to;
        this.cost = cost;
    }
    public AbstractEdge(int from, int to) {
        this(from, to, 1l);
    }
    public abstract AbstractEdge reverse();
    public int compareTo(AbstractEdge o) {
        return Long.compare(cost, o.cost);
    }
}
/**
 * @author https://atcoder.jp/users/suisen
 */
class Graph<Edg extends AbstractEdge> extends AbstractGraph<Edg> {
    public Graph(int n) {
        super(n);
    }
    @SuppressWarnings("unchecked")
    @Override
    public void addEdge(Edg edge) {
        Edg rev = (Edg) edge.reverse();
        adj.get(edge.from).add(edge);
        adj.get(edge.to).add(rev);
        edges.add(edge);
    }
}