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

long long count_inversions(vector<int>& v) {
    long long inv = 0;
    int n = v.size();
    for(int i = 0; i < n; i++)
        for(int j = i+1; j < n; j++)
            if(v[i] > v[j])
                inv++;
    return inv;
}

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());

    long long ans = 0;

    for(int start = 0; start < k; start++) {

        vector<int> cur, need;

        for(int i = start; i < n; i += k) {
            cur.push_back(a[i]);
            need.push_back(b[i]);
        }

        vector<int> t1 = cur, t2 = need;
        sort(t1.begin(), t1.end());
        sort(t2.begin(), t2.end());

        if(t1 != t2) {
            cout << -1;
            return 0;
        }

        ans += count_inversions(cur);
    }

    cout << ans;
}