import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        try (Scanner scan = new Scanner(System.in)) {
            int n = scan.nextInt();
            int k = scan.nextInt();
            int list[] = new int[n];
            for(int i=0; i<n; i++) {
                list[i] = scan.nextInt();
            }
            
            int ans = 0;
            int m = n;
            while(m > 0) {
                for(int g=0; g<k; g++) {
                    for(int p=g; p+k<m; p+=k) {
                        if(list[p] > list[p+k]) {
                            int w = list[p];
                            list[p] = list[p+k];
                            list[p+k] = w;
                            ans++;
                        }
                    }
                }
                m -= k;
            }
                        
            boolean flag = true; 
            for(int i=0; i<n-1; i++) {
                if(list[i] > list[i+1]) {
                    flag = false;
                    break;
                }
            }
            System.out.println(flag? ans : -1);
        }
    }
}