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

const int mod = 1e9+7;
const int maxn = 1e5 + 1;
#define pii pair<int, int>
#define ll long long
#define pq priority_queue
#define se second
#define fi first
int n, k;
vector<int> a;
bool dasap(vector<int> &ans) {
    for (int i = 0; i < ans.size()- 1; i++) {
        if(a[i] > a[i + 1]) return false;
    }
    return true;
}
void solve() {
    cin >> n;
    a.resize(n);
    for (int i = 0; i < n; i++) cin >> a[i];
    cin >> k;
    int ans = 0;
    for (int it = 0; it <= 10000; it++) {
        for (int i = 0; i < n - k; i++) {
            if(a[i] > a[i + k]) {
                swap(a[i], a[k + i]);
                ans++;
            }
        }
    }
    if(!dasap(a)) cout << -1 << '\n';
    else cout << ans << '\n';
}

signed main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);
    
    solve();
    return 0;
}