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

const int INF = 2e9;
const int MN = 105;
const int MOD = 1000000007;
const int LOG = 20;

int n,k;
int a[MN];

long long merge(vector<int>& v, int l, int r) {
    if (l >= r) return 0;

    int mid = l + (r - l) / 2;
    long long count = 0;

    count += merge(v, l, mid);
    count += merge(v, mid + 1, r);

    vector<int> tmp;
    int i = l, j = mid + 1;
    
    while (i<=mid && j<=r) {
        if (v[i] <= v[j]) {
            tmp.push_back(v[i]);
            i++;
        } else {
            tmp.push_back(v[j]);
            count += (mid - i + 1);
            j++;
        }
    }

    while (i <= mid) tmp.push_back(v[i++]);
    while (j <= r) tmp.push_back(v[j++]);

    for (int p = 0; p < tmp.size(); p++) {
        v[l + p] = tmp[p];
    }

    return count;
}

int main() {
	ios_base::sync_with_stdio(false);
	cin.tie(0); cout.tie(0);
	
	int ans = 0;
	cin >> n;
	for (int i=1; i<=n; i++){
		cin >> a[i];
	}
	cin >> k;
	vector<int> g[k];
	for (int i=1; i<=n; i++){
		g[i%k].push_back(a[i]);
	}
	for (int i=0; i<k; i++) {
		if (g[i].empty()) continue;
		int m = g[i].size();
		ans += merge(g[i],0,m-1);
	}
	for (int i=0; i<k; i++){
		for (int j=0; j<g[i].size(); j++){
			if (i!=0) a[j*k+i] = g[i][j];
			else a[(j+1)*k] = g[i][j];
		}
	}
	for (int i=2; i<=n; i++){
		if (a[i]<a[i-1]) {
			cout << -1;
			return 0;
		}
	}
	cout << ans;
	
    return 0;
}