import java.util.Arrays;
import java.util.Scanner;

public class Yuki366 {
	static Scanner sc = new Scanner(System.in);

	public static void main(String[] args) {
		int N, K;
		N = sc.nextInt();
		K = sc.nextInt();
		int[] A = new int[N];
		int[] sortA = new int[N];
		for (int i = 0; i < N; i++) {
			A[i] = sc.nextInt();
			sortA[i] = A[i];
		}
		Arrays.sort(sortA);
		int ans = Ksort(A, K);
		if (!Arrays.equals(A, sortA)) {
			ans = -1;
		}
		System.out.println(ans);
	}

	public static int Ksort(int[] A, int K) {
		int cnt = 0;
		for (int i = 0; i < A.length; i++) {
			for (int j = 0; j + K < A.length; j++) {
				if (A[j] > A[j + K]) {
					int buf = A[j];
					A[j] = A[j + K];
					A[j + K] = buf;
					cnt++;
				}
			}
		}
		return cnt;
	}
}