#include <iostream>
#include <vector>
#include <algorithm>
#include <iterator>
#include <utility>

using namespace std;

int main() {

    ios::sync_with_stdio(false);
    cin.tie(0);

    int n, k;
    cin >> n >> k;
    vector<int> rawVec(n);
    for (auto& i : rawVec) {
        cin >> i;
    }
    if ( is_sorted(rawVec.begin(), rawVec.end())) {
        cout << "0" << endl;
        return 0;
    } else {
        if (n <= k) {
            cout << "-1" << endl;
            return 0;
        } else {
            vector<int> sortedVec = rawVec;
            sort(sortedVec.begin(), sortedVec.end());
            for (size_t i = 0; i < sortedVec.size(); ++i) {
                auto itr = find(rawVec.begin(), rawVec.end(), sortedVec.at(i));
                size_t rawDist = distance(rawVec.begin(), itr);
                if (((int)rawDist - (int)i) % k != 0) {
                    cout << "-1" << endl;
                    return 0;
                }
            }
            int ans = 0;
            for (int i = 0; i < n; ++i) {
                for (int j = 0; j + k < n; ++j) {
                    if (rawVec.at(j) > rawVec.at(j + k)) {
                        swap(rawVec.at(j), rawVec.at(j + k));
                        ans++;
                    }
                }
            }
            cout << ans << endl;
            return 0;
        }
    }
    cout << "nandeyanen" << endl;
    return 0;
}