#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;
struct node{
    int id, val;
};
vector<node> a;

void solve() {
    cin >> n;
    a.resize(n);
    for (int i = 0; i < n; i++){
        cin >> a[i].val;
        a[i].id = i;
    }
    sort(a.begin(), a.end(), [](const node &x, const node &y) {
        return x.val < y.val;
    });
    cin >> k;
    bool ok = true;
    int ans = 0;
    for (int i = 0; i < n; i++) {
        if((a[i].id - i)%k != 0) {
            ok = false;
            break;
        }
        ans += abs((a[i].id - i)/k);
    }
    if(!ok) cout << -1 << '\n';
    else cout << ans/2 << '\n';
}

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