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

public class Main {
    
    private static int[] dy = {0,1,0,-1};
    private static int[] dx = {1,0,-1,0};
    private static BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    
    public static void main(String[] args) throws Exception {

    	int[] n_k = readIntArray();
    	int n = n_k[0];
    	int k = n_k[1];
    	
    	int[] aArr = readIntArray();

    	int count = 0;
    	if(k < n){
        	for(int i = 0 ; i < n ; i++){
        	    for(int j = i + k ; j < n ; j = j + k){
        	        if(aArr[i] > aArr[j]){
        	            int tmp = aArr[i];
        	            aArr[i] = aArr[j];
        	            aArr[j] = tmp;
        	            count++;
        	        }
        	    }
        	}
    	}
    	
    	for(int i = 0 ; i < n - 1 ; i++){
    	    if(aArr[i] > aArr[i+1]){
    	        System.out.println(-1);
    	        return;
    	    }
    	}
    	
        System.out.println(count);
		
    }
    
    
    private static int[] readIntArray() throws Exception{
        
		int[] intArray = Arrays.stream(br.readLine().split(" "))
							.mapToInt(Integer::parseInt).toArray();

		return intArray;

	}

}