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

public class Main {
	static int cnt = 0;
	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		int N = scan.nextInt();
		int K = scan.nextInt();
		int []a = new int[N];
		int []b = new int[N];
		for(int i = 0; i < N; i++) {
			a[i] = scan.nextInt();
			b[i] = a[i];
		}
		scan.close();
		Arrays.sort(b);
		for(int i = 0; i < K; i++) {
			kStepBubbleSort(a, N - i, K);
		}
		for(int i = 0; i < N; i++) {
			if(a[i] != b[i]) {
				System.out.println("-1");
				System.exit(0);
			}
		}
		System.out.println(cnt);
//		for(int i : a) {
//			System.out.print(i + " ");
//		}
//		System.out.println();
	}
	static void kStepBubbleSort(int[]A, int n, int K) {
		boolean flag = true;
		while(flag) {
			flag = false;
			for(int i = n - 1; i - K >= 0; i -= K) {
				if(A[i] < A[i - K]) {
					int t = A[i];
					A[i] = A[i - K];
					A[i - K] = t;
					flag = true;
					cnt ++;
				}
			}
		}
	}
}