#include <iostream>
#include <iomanip>
#include <vector>
#include <algorithm>
#include <string>
#include <sstream>
#include <cmath>
#include <stack>
#include <queue>
#include <cctype>
#include <stdio.h>
#include <map>
#include <string.h>
#include <utility>

typedef long long ll;
using namespace std;

typedef pair<int, int> P;

ll block[1001];
int main() {

	int n,k;
	cin>>n>>k;

	for(int i=0; i<n; i++){

		ll tmp=0;
		cin>>tmp;
		block[i] =tmp;
	}

	bool judge=true;

	int cp=0;

	while(judge){
		judge =false;

		for(int i=0; i<n-k; i++){
			if(block[i] > block[i+k]){
				ll tmp = block[i+k];
				block[i+k] = block[i];
				block[i] =tmp;

				judge =true;
				cp++;
			}
		}
	}

		for(int i=0; i<n-1; i++){
			if(block[i] > block[i+1]){
				cp =-1;
				break;
			}
		}



	cout<<cp<<endl;


	return 0;
}