#include <bits/stdc++.h>
using namespace std;

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

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

    vector<int> a(n);
    for (int i = 0; i < n; i++)
        cin >> a[i];

    vector<int> b = a;
    sort(b.begin(), b.end());

    vector<vector<int>> groups(k);

    // chia nhóm
    for (int i = 0; i < n; i++)
        groups[i % k].push_back(a[i]);

    // sort từng nhóm
    for (int i = 0; i < k; i++)
        sort(groups[i].begin(), groups[i].end());

    // ghép lại
    vector<int> rebuilt(n);
    vector<int> idx(k, 0);

    for (int i = 0; i < n; i++) {
        int g = i % k;
        rebuilt[i] = groups[g][idx[g]++];
    }

    // check khả thi
    if (rebuilt != b) {
        cout << -1;
        return 0;
    }

    long long ans = 0;

    // đếm swap từng nhóm
    for (int i = 0; i < k; i++) {
        int sz = groups[i].size();
        int correct = 0;

        for (int j = 0; j < sz; j++) {
            if (groups[i][j] == b[i + j*k])
                correct++;
        }

        ans += (sz - correct);
    }

    cout << ans / 2;
}