package yukicoder_7273;

import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.InputStream;
import java.util.ArrayList;
import java.util.BitSet;
import java.util.Comparator;
import java.util.NoSuchElementException;
import java.util.PriorityQueue;
import java.util.TreeMap;

public class Main {

	public static void main(String[] args) {
		new Main();
	}

	public Main() {
		InputChecker ic = new InputChecker(System.in);
		java.io.PrintWriter out = new java.io.PrintWriter(System.out);
		solve(ic, out);
		out.flush();
	}

	public void solve(InputChecker ic, java.io.PrintWriter out) {
		int N = ic.nextInt(3, 500);
		ic.nextChar(' ');
		int M = ic.nextInt(0, exponent10(1, 5));
		ic.readNewLine();

		int[] u = new int[M], v = new int[M];
		for (int i = 0;i < M;++ i) {
			u[i] = ic.nextInt(1, N) - 1;
			ic.nextChar(' ');
			v[i] = ic.nextInt(u[i] + 2, N) - 1;
			ic.readNewLine();
		}
		ic.checkEOF();
		ArrayList<TreeMap<Integer, Edge>> graph = new ArrayList<>(N);
		for (int i = 0;i < N;++ i) graph.add(new TreeMap<>());
		PriorityQueue<Edge> check = new PriorityQueue<>();
		for (int i = 0;i < M;++ i) {
			Edge e = new Edge(u[i], v[i]);
			graph.get(e.u).put(e.dist(e.u, N), e);
			graph.get(e.v).put(e.dist(e.v, N), e);
		}
		for (int i = 0;i < N;++ i) {
			Edge e = new Edge(i, (i + 1) % N);
			e.outerDistance = 0;
			graph.get(e.u).put(e.dist(e.u, N), e);
			graph.get(e.v).put(e.dist(e.v, N), e);
			check.add(e);
		}
		for (int i = 0;i < N;++ i) {
			if (graph.get(i).size() % 2 != 0) {
				out.println(5); // 面が二部グラフではない⇔Gがオイラーグラフではない
				return;
			}
		}
		while(!check.isEmpty()) {
			Edge e = check.poll();
			if (e.remove) continue;
			int now = e.u;
			if (graph.get(now).lastEntry().getValue().getTarget(now) != e.v) now = e.v;
			int move = N - e.dist(e.getTarget(now), N), distance = e.outerDistance, face = 1;
			graph.get(e.u).remove(e.dist(e.u, N));
			graph.get(e.v).remove(e.dist(e.v, N));
			while(move > 0) {
				e = graph.get(now).floorEntry(move).getValue();
				if (e.outerDistance < 0) {
					e.outerDistance = distance + 1;
					check.add(e);
				} else {
					graph.get(e.u).remove(e.dist(e.u, N));
					graph.get(e.v).remove(e.dist(e.v, N));
					e.remove = true;
				}
				++ face;
				move -= e.dist(now, N);
				now = e.getTarget(now);
			}
			if (distance % 2 == 0 && face % 2 != 0) {
				out.println(5); // ある内平面が奇閉路で構成されていて、かつ外平面と異なる色で塗られる
				return;
			}
		}
		out.println(4);
	}
	static class Edge implements Comparable<Edge>{
		int u, v;
		int outerDistance = -1;
		boolean remove = false;
		Edge(int u, int v) {
			this.u = u;
			this.v = v;
		}
		int getTarget(int source) {
			return source ^ u ^ v;
		}
		int dist(int source, int N) {
			return getTarget(source) < source ? N + getTarget(source) - source : getTarget(source) - source;
		}
		static Comparator<Edge> comparator(int source, int N) {
			return (l, r) -> Integer.compare(r.dist(source, N), l.dist(source, N));
		}
		@Override
		public int compareTo(Edge o) {
			return Integer.compare(outerDistance, o.outerDistance);
		}
		@Override
		public String toString() {
			return "{" + u + "," + v + "}";
		}
	}

	public static class LongSet {

	}

	public static int exponent10(int n, int e) {
		return n * pow(10, e);
	}

	public static long exponent10L(int n, int e) {
		return n * pow(10L, e);
	}

	public static int pow(int a, int b) {
		int ans = 1;
		for (int mul = a;b > 0;b >>= 1, mul *= mul) if ((b & 1) != 0) ans *= mul;
		return ans;
	}

	public static long pow(long a, long b) {
		long ans = 1;
		for (long mul = a;b > 0;b >>= 1, mul *= mul) if ((b & 1) != 0) ans *= mul;
		return ans;
	}

	public static class CharSet {
		private final BitSet set = new BitSet(256);

		public void add(char... c) {
			for (char i : c) set.set(i);
		}

		public void add(CharSet... s) {
			for (CharSet i : s) set.or(i.set);
		}

		public boolean contains(char... c) {
			for (char i : c) if (!set.get(i)) return false;
			return true;
	 	}

		public boolean contains(String s) {
			return contains(s.toCharArray());
	 	}

		private static final class Chars extends CharSet {
			public Chars(char... c) {
				super.add(c);
			}
			public Chars(CharSet... s) {
				super.add(s);
			}
			@Override
			public void add(char... c) {
				throw new UnsupportedOperationException();
			}
			@Override
			public void add(CharSet... s) {
				throw new UnsupportedOperationException();
			}
		}

		public static final CharSet NUMBER = new Chars('0','1','2','3','4','5','6','7','8','9');
		public static final CharSet LOWER = new Chars('a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z');
		public static final CharSet UPPER = new Chars('A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z');
		public static final CharSet ALPHABET = new Chars(LOWER, UPPER);

	}
	public static class InputChecker {
		private InputStream in;
		private final byte[] buffer = new byte[1024];
		private final byte[] undo = new byte[1024];
		private int undoSize = 0;
		private int read = 0;
		private int length = 0;

		public InputChecker(InputStream in) {
			this.in = in;
		}

		public final void setInputStream(InputStream in) {
			this.in = in;
		}

		public final void setInputStream(File in) {
			try {
				this.in = new FileInputStream(in);
			} catch (FileNotFoundException e) {
				e.printStackTrace();
			}
		}

		private boolean hasNextByte() {
			if (undoSize > 0) return true;
			if (read < length) return true;
			read = 0;
			try {
				length = in.read(buffer);
			} catch (IOException e) {
				e.printStackTrace();
			}
			return length > 0;
		}

		private byte readByte() {
			if (hasNextByte()) return undoSize > 0 ? undo[--undoSize] : buffer[read++];
			throw new NoSuchElementException();
		}

		private void undo(byte b) {
			undo[undoSize ++] = b;
		}

		private void undo(char c) {
			if ((c & 0xFF80) == 0) {
				undo((byte)c);
				return;
			}
			undo((byte)(c & 0x3F | 0x80));
			if ((c & 0xF800) == 0) {
				undo((byte)(c >> 6 & 0x1F | 0xC0));
			} else {
				undo((byte)(c >> 6 & 0x3F | 0x80));
				undo((byte)(c >> 12 | 0xE0));
			}
		}

		public final boolean hasNext() {
			return hasNextByte();
		}

		public final char nextChar() {
			byte b = readByte();
			if ((b & 0x80) == 0) return (char)b;
			if ((b & 0x20) == 0) return (char)((b & 0x1F) << 6 | readByte() & 0x3F);
			return (char)((b & 0xF) << 12 | (readByte() & 0x3F) << 6 | readByte() & 0x3F);
		}

		public final char nextChar(char estimate) {
			char c = nextChar();
			if (estimate == c) return estimate;
			undo(c);
			throw new AssertionError();
		}

		public final char nextChar(CharSet estimate) {
			char c = nextChar();
			if (estimate.contains(c)) return c;
			undo(c);
			throw new AssertionError();
		}

		public final void readNewLine() {
			char c = nextChar();
			if (c == '\r') {
				c = nextChar();
				if (c != '\n') undo(c);
				return;
			} else if (c == '\n') return;
			undo(c);
			throw new AssertionError();
		}

		public final void checkEOF() {
			if (hasNextByte()) throw new AssertionError();
		}

		public final String next(CharSet contains) {
			StringBuilder sb = new StringBuilder();
			try {
				do {
					char c = nextChar();
					if (!contains.contains(c)) {
						undo(c);
						return sb.toString();
					}
					sb.append(c);
				} while(true);
			} catch (NoSuchElementException e) {
				if (sb.length() <= 0) throw new AssertionError();
				return sb.toString();
			}
		}

		public final int nextInt() {
			byte b = readByte();
			int n = 0;
			if (b == '-') {
				if (!isNumber(b = readByte())) {
					undo(b);
					throw new NumberFormatException();
				}
				try {
					if (b == '0') {
						if (!isNumber(b = readByte())) {
							undo(b);
							return 0;
						}
						throw new NumberFormatException();
					}
					do n = Math.addExact(Math.multiplyExact(n, 10), '0' - b); while(isNumber(b = readByte()));
					undo(b);
				} catch (NoSuchElementException e) {
				}
				return n;
			}
			if (!isNumber(b)) {
				undo(b);
				throw new NumberFormatException();
			}
			try {
				if (b == '0') {
					if (!isNumber(b = readByte())) {
						undo(b);
						return 0;
					}
					throw new NumberFormatException();
				}
				do n = Math.addExact(Math.multiplyExact(n, 10), b - '0'); while(isNumber(b = readByte()));
				undo(b);
			} catch (NoSuchElementException e) {
			}
			return n;
		}

		public final int nextInt(int min, int max) {
			int n = nextInt();
			if (min <= n && n <= max) return n;
			throw new NumberFormatException();
		}

		private static boolean isNumber(byte c) {
			return '0' <= c && c <= '9';
		}

		public final long nextLong() {
			byte b = readByte();
			long n = 0;
			if (b == '-') {
				if (!isNumber(b = readByte())) {
					undo(b);
					throw new NumberFormatException();
				}
				try {
					if (b == '0') {
						if (!isNumber(b = readByte())) {
							undo(b);
							return 0;
						}
						throw new NumberFormatException();
					}
					do n = Math.addExact(Math.multiplyExact(n, 10), '0' - b); while(isNumber(b = readByte()));
					undo(b);
				} catch (NoSuchElementException e) {
				}
				return n;
			}
			if (!isNumber(b)) {
				undo(b);
				throw new NumberFormatException();
			}
			try {
				if (b == '0') {
					if (!isNumber(b = readByte())) {
						undo(b);
						return 0;
					}
					throw new NumberFormatException();
				}
				do n = Math.addExact(Math.multiplyExact(n, 10), b - '0'); while(isNumber(b = readByte()));
				undo(b);
			} catch (NoSuchElementException e) {
			}
			return n;
		}

		public final long nextLong(long min, long max) {
			long n = nextLong();
			if (min <= n && n <= max) return n;
			throw new NumberFormatException();
		}

		public final double nextDouble() {
			StringBuilder sb = new StringBuilder();
			byte b = readByte();
			if (b == '-') {
				sb.append(b);
				b = readByte();
			}
			if (b == '0') {
				sb.append(b);
				b = readByte();
			} else {
				while(isNumber(b)) {
					sb.append(b);
					b = readByte();
				}
			}
			if (b == '.') {
				sb.append(b);
				b = readByte();
				while(isNumber(b)) {
					sb.append(b);
					b = readByte();
				}
			}
			if (b == 'e' || b == 'E') {
				sb.append(b);
				b = readByte();
				if (b == '-' || b == '+') {
					sb.append(b);
					b = readByte();
				}
				while(isNumber(b)) {
					sb.append(b);
					b = readByte();
				}
			}
			undo(b);
			return Double.parseDouble(sb.toString());
		}
	}
}