/* package whatever; // don't place package name! */

import java.util.*;
import java.lang.*;
import java.io.*;

/* Name of the class has to be "Main" only if the class is public. */
class Ideone
{
    public static void main (String[] args) throws java.lang.Exception
    {
        // your code goes here
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        String[] lines = br.readLine().split(" ");
        int n = Integer.parseInt(lines[0]);
        int k = Integer.parseInt(lines[1]);
        lines = br.readLine().split(" ");
        
        int[] arr = new int[n];
        int idx = 0;
        for(String st:lines){
            arr[idx++] = Integer.parseInt(st);
        }

        int tmp,ans = 0;
        for(int i=0;i<n-k;i++){
            if(arr[i] > arr[i+k]){
                tmp = arr[i];
                arr[i] = arr[i+k];
                arr[i+k] = tmp;
                i=-1;
                ++ans;
            }
        }
        tmp = -1;
        for(int i:arr){
            if(tmp>i){
                ans = -1;
                break;
            }
            tmp = i;
        }
        System.out.println(ans);
    }
}