import java.io.BufferedInputStream;
import java.util.Arrays;
import java.util.Scanner;
public class Main {
	static final long C =  1000000007;
	static final int CY = 1000000000;
	StringBuilder sb;

	public void calc() {
		sb = new StringBuilder();
		BufferedInputStream bs = new BufferedInputStream(System.in);

		Scanner sc = new Scanner(bs);

		int n = sc.nextInt();
		int k = sc.nextInt();

		int[][] a = new int[k][1+n/k];
		for (int i=0; i < n; i++) {
			a[i%k][i/k] = sc.nextInt();
		}


		long ans = 0;
		for (int v = 0; v < k; v++) {
			for (int i=0; i < a[v].length; i++) {
				for (int t=0; t <i; t++) {
					if (a[v][i] == 0) break;
					if (a[v][i] < a[v][t]) ans++;
				}
			}
			Arrays.sort(a[v]);
		}
		for (int i=1; i < n; i++) {
			if (a[(i-1)%k][(i-1)/k] > a[i%k][i/k]){ans = -1;break;}
		}

		System.out.println(ans);
	}

	public static void main(String[] args) {
		Main main = new Main();
		main.calc();

	}
}