#include <bits/stdc++.h>
using namespace std;
int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    int n, k;
    cin >> n >> k;
    vector<long long> a(n);
    for (int i = 0; i < n; i++) cin >> a[i];
    vector<long long> b = a;
    sort(b.begin(), b.end());
    long long ans = 0;
    for (int start = 0; start < k; start++) {
        vector<long long> cur, need;
        for (int i = start; i < n; i += k) {
            cur.push_back(a[i]);
            need.push_back(b[i]);
        }
        auto t1 = cur, t2 = need;
        sort(t1.begin(), t1.end());
        sort(t2.begin(), t2.end());
        if (t1 != t2) {
            cout << -1;
            return 0;
        }
        int m = cur.size();
        for (int i = 0; i < m; i++) {
            for (int j = 0; j + 1 < m; j++) {
                if (cur[j] > cur[j + 1]) {
                    swap(cur[j], cur[j + 1]);
                    ans++;
                }
            }
        }
    }
    cout << ans;
}