#include <bits/stdc++.h>
using namespace std;
int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    int n;
    cin >> n;
    vector<long long> a(n);
    for (int i = 0; i < n; i++) {
        cin >> a[i];
    }
    int k;
    cin >> k;
    vector<long long> b = a;
    sort(b.begin(), b.end());
    int count = 0;
    for (int i = 0; i < k; i++) {
        for (int j = i; j < n - k; j+=k) {
        	if (a[i] > a[i + k]) {
         	    swap(a[i], a[i + k]);
                count++;
            }
        }
    }
    if (a != b) {
        cout << -1;
    } else {
        cout << count;
    }
    return 0;
}