#include <algorithm>
#include <climits>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <functional>
#include <iostream>
#include <map>
#include <ostream>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <vector>
#define INF 1000000000
#define rep(i,a,b) for (int i=(a);i<(b);i++)
#define rev(i,a,b) for (int i=(a)-1;i>=b;i--)
using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
typedef queue<int> qi;
typedef vector<int> vi;
typedef vector<string> vs;
int dx[4] = { 0, 1, 0, -1 };
int dy[4] = { 1, 0, -1, 0 };

int n, k;
ll a[1000];

int main() {
	cin.tie(0);
	ios::sync_with_stdio(false);

	cin >> n >> k;
	rep(i,0,n) cin >> a[i];

	ll ans = 0;
	rep(i,0,k) {
		for(int j = i; j < n; j+=k) {
			for(int l = j+k; l < n; l+=k) {
				if(a[j] > a[l]) {
					ll tmp = a[j];
					a[j] = a[l];
					a[l] = tmp;
					ans++;
				}
			}
		}
	}

	// rep(i,0,n) cout << a[i] << " ";
	// cout << endl;

	rep(i,1,n)
		if(a[i-1] > a[i]) {
			cout << "-1" << endl;
			return 0;
		}
	cout << ans << endl;

	return 0;
}