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];
		HashMap<Integer, Integer> map = new HashMap<>();
		for (int i = 0; i < n; i++) {
		    arr[i] = sc.nextInt();
		    map.put(arr[i], i);
		}
		int[] sorted = (int[])(arr.clone());
		Arrays.sort(sorted);
		int count = 0;
		for (int i = 0; i < n - 1; i++) {
		    int idx = map.get(sorted[i]);
		    if (i == idx) {
		        continue;
		    }
		    if ((idx - i) % k != 0) {
		        System.out.println(-1);
		        return;
		    }
		    while (i < idx) {
		        int tmp = arr[idx - k];
		        arr[idx - k] = arr[idx];
		        arr[idx] = tmp;
		        map.put(sorted[i], idx - k);
		        map.put(arr[idx], idx);
		        idx -= k;
		        count++;
		    }
		}
	    System.out.println(count);
	}
}