#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define llong long long

int swap_cnt = 0;

void swapping(vector<int> &val, int len) {
    rep(i, len-1) {
        rep(j, len-1-i) {
            if(val[j] > val[j+1]) {
                swap(val[j], val[j+1]);
                swap_cnt++;
            }
        }
    }
}

int main() {
    int n, k;
    cin >> n >> k;
    vector<int> a(n);
    rep(i, n) cin >> a[i];

    vector<vector<int>> sp(k, vector<int>());
    rep(i, n) sp[i%k].push_back(a[i]);
    sort(a.begin(), a.end());
    rep(i, k) swapping(sp[i], sp[i].size());
    rep(i, n) {
        if(a[i] != sp[i%k][i/k]) {
            cout << -1 << "\n";
            return 0;
        }
    }
    cout << swap_cnt << "\n";
}