import java.io.IOException;
import java.io.InputStream;
import java.util.NoSuchElementException;

public class Main {
    FastScanner fs = new FastScanner();

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

    void solve() {
        int N = fs.nextInt();
        int Width = fs.nextInt();
        int[] a = new int[N];
        int Count = 0;
        for(int i = 0; i < N; i++) {
            a[i] = fs.nextInt();
        }
        if(a.length == 1) {
            System.out.println(0);
            return;
        }
        int i = 0;
        int loopCount = 0;
        if((i + Width) < N) {
            while (true) {
                if (a[i] > a[i + Width]) {
                    int Temp = a[i + Width];
                    a[i + Width] = a[i];
                    a[i] = Temp;
                    Count++;
                    loopCount++;
                }
                i++;
                if (loopCount == 0) {
                    break;
                }
                if (i == (a.length - Width)) {
                    i = 0;
                    loopCount = 0;
                }
            }
        }
        for(int j = 0; j < N; j++) {
            if(a[j] > a[j + 1]) {
                System.out.println(-1);
                return;
            }
        }
        System.out.println(Count);
    }
}

class FastScanner {
    private final InputStream in = System.in;
    private final byte[] buffer = new byte[1024];
    private int ptr = 0;
    private int buflen = 0;

    private boolean hasNextByte() {
        if (ptr < buflen) {
            return true;
        } else {
            ptr = 0;
            try {
                buflen = in.read(buffer);
            } catch (IOException e) {
                e.printStackTrace();
            }
            if (buflen <= 0) {
                return false;
            }
        }
        return true;
    }

    private int readByte() {
        if (hasNextByte()) return buffer[ptr++];
        else return -1;
    }

    private static boolean isPrintableChar(int c) {
        return 33 <= c && c <= 126;
    }

    private void skipUnprintable() {
        while (hasNextByte() && !isPrintableChar(buffer[ptr])) ptr++;
    }

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

    public String next() {
        if (!hasNext()) throw new NoSuchElementException();
        StringBuilder sb = new StringBuilder();
        int b = readByte();
        while (isPrintableChar(b)) {
            sb.appendCodePoint(b);
            b = readByte();
        }
        return sb.toString();
    }

    public long nextLong() {
        if (!hasNext()) throw new NoSuchElementException();
        long n = 0;
        boolean minus = false;
        int b = readByte();
        if (b == '-') {
            minus = true;
            b = readByte();
        }
        if (b < '0' || '9' < b) {
            throw new NumberFormatException();
        }
        while (true) {
            if ('0' <= b && b <= '9') {
                n *= 10;
                n += b - '0';
            } else if (b == -1 || !isPrintableChar(b)) {
                return minus ? -n : n;
            } else {
                throw new NumberFormatException();
            }
            b = readByte();
        }
    }

    public int nextInt() {
        if (!hasNext()) throw new NoSuchElementException();
        int n = 0;
        boolean minus = false;
        int b = readByte();
        if (b == '-') {
            minus = true;
            b = readByte();
        }
        if (b < '0' || '9' < b) {
            throw new NumberFormatException();
        }
        while (true) {
            if ('0' <= b && b <= '9') {
                n *= 10;
                n += b - '0';
            } else if (b == -1 || !isPrintableChar(b)) {
                return minus ? -n : n;
            } else {
                throw new NumberFormatException();
            }
            b = readByte();
        }
    }
}