import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int k = sc.nextInt();
        int[] arr = new int[n];
        for (int i = 0; i < n; i++) {
            arr[i] = sc.nextInt();
        }
        int count = 0;
        for (int i = 0; i < k; i++) {
            for (int j = n; i + k < j; j -= k) {
                for (int a = i; a + k < j; a += k) {
                    if (arr[a] > arr[a + k]) {
                        int tmp = arr[a];
                        arr[a] = arr[a + k];
                        arr[a + k] = tmp;
                        count++;
                    }
                }
            }
        }
        for (int i = 0; i < n - 1; i++) {
            if (arr[i] > arr[i + 1]) {
                System.out.println(-1);
                return;
            }
        }
        System.out.println(count);
    }
}