#include <iostream>
#include <vector>

using namespace std;

int aritmeticki_trougao(const vector<int> &a, int k)
{
    int p = 1;
    int b = 1;

    for (int i = 1; i < k; i++) {
        p = p + b;
        b = b + 2;
    }

    int zbir = 0;
    for (int i = p; i < p + b; i++) {
        zbir += i;
    }

    return zbir;
}

int main(void)
{
    int n, k;

    vector<int> a;

    cin >> n;
    a.resize(n);
    for (int i = 0; i < n; i++) {
        cin >> a[i];
    }

    cin >> k;

    cout << aritmeticki_trougao(a, k) << endl;

    
    return 0;
}