import java.io.*;
import java.util.*;


public class Main_yukicoder366 {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        Printer pr = new Printer(System.out);

        int n = sc.nextInt();
        int k = sc.nextInt();
        int[] a = new int[n];
        for (int i = 0; i < n; i++) {
        	a[i] = sc.nextInt();
        }

        int cnt = 0;
        boolean flag = true;
        while (flag) {
        	flag = false;
        	for (int i = 0; i < n; i++) {
        		if (i + k >= n) {
        			break;
        		}

        		if (a[i] > a[i + k]) {
        			int tmp = a[i];
        			a[i] = a[i + k];
        			a[i + k] = tmp;
        			cnt++;
        			flag = true;
        		}
        	}
        }

        boolean sorted = true;
        for (int i = 0; i < n - 1; i++) {
        	if (a[i] > a[i + 1]) {
        		sorted = false;
        		break;
        	}
        }

        if (sorted) {
        	pr.println(cnt);
        } else {
        	pr.println(-1);
        }

        pr.close();
        sc.close();
    }

	@SuppressWarnings("unused")
	private static class Scanner {
		BufferedReader br;
		Iterator<String> it;

		Scanner (InputStream in) {
			br = new BufferedReader(new InputStreamReader(in));
		}

		String next() throws RuntimeException  {
			try {
				if (it == null || !it.hasNext()) {
					it = Arrays.asList(br.readLine().split(" ")).iterator();
				}
				return it.next();
			} catch (IOException e) {
				throw new IllegalStateException();
			}
		}

		int nextInt() throws RuntimeException {
			return Integer.parseInt(next());
		}

		long nextLong() throws RuntimeException {
			return Long.parseLong(next());
		}

		float nextFloat() throws RuntimeException {
			return Float.parseFloat(next());
		}

		double nextDouble() throws RuntimeException {
			return Double.parseDouble(next());
		}

		void close() {
			try {
				br.close();
			} catch (IOException e) {
//				throw new IllegalStateException();
			}
		}
	}

	private static class Printer extends PrintWriter {
		Printer(PrintStream out) {
			super(out);
		}
	}
}