#include <iostream>
#include <string>
#include <algorithm>
#include <functional>
#include <vector>
#include <stack>
#include <queue>
#include <set>
#include <bitset>
#include <map>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
using namespace std;
typedef long long ll;
typedef pair<int,int> pint;
typedef vector<int> vint;
typedef vector<pint> vpint;
#define mp make_pair
#define fi first
#define se second
#define all(v) (v).begin(),(v).end()
#define rep(i,n) for(int i=0;i<(n);i++)
#define REP(i,n) for (int i=(n)-1;i>=0;i--)
#define reps(i,f,n) for(int i=(f);i<(n);i++)
#define REPS(i,f,n) for (int i=(f)-1;i>=(n);i--)
int dx[4] = {1, 0, 0, -1};
int dy[4] = {0, 1, -1, 0};


int main(void){
	int N, k;
	scanf("%d %d", &N, &k);
	vector<int> a(N);

	rep(i, N){
		scanf("%d", &a[i]);
	}

	int cnt = 0;
	int flag = 0;
	do{
		flag = 0;
		rep(i, N - k){
			if (a[i] > a[i + k]){
				swap(a[i], a[i + k]);
				flag = 1;
				cnt++;
			}
		}

	}while(flag != 0);

	//順番に並んでいるかを確かめる
	rep(i, N - 1){
		if (a[i] > a[i + 1]){
			printf("-1\n");
			return 0;
		}
	}

	printf("%d\n", cnt);

	return 0;
}