#include<bits/stdc++.h>
using namespace std;
const int N = 305;
int a[N];
pair<int,pair<int,int>> b[N];
int main(){
    int n , k;
    bool kt = true;
    cin >> n;
    for(int i = 1;  i <= n ; i ++){
        cin >> a[i];
        b[i].first = a[i];
        b[i].second.first = i;
    }
    cin >> k;
    sort(b + 1, b + n + 1);
    int x = 0;
    for(int i = 1 ; i <= n ; i ++){
        b[i].second.second = i;
    }
    for(int i = 1; i <= n ;i ++){
        if(abs(b[i].second.second - b[i].second.first) % k == 0){
            if(b[i].second.second != b[i].second.first){
                x += abs(b[i].second.second - b[i].second.first) / k;
            }
        }
        else{
            kt = false;
        }
    }
    if(kt){
        cout << x / 2;
    }
    else{
        cout << -1;
    }
}