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

int main() {
    int64_t n, m;
    cin >> n;
    vector<int64_t> x(n);
    for (int i = 0; i < n; i++) {
        cin >> x.at(i);
    }
    cin >> m;
    int64_t s = 0, s_square = 0;
    for (int i = 0; i < n; i++) {
        s += x.at(i);
        s_square += x.at(i) * x.at(i);
    }
    int64_t var_n2 = n * s_square - s * s;
    if (n * s_square == s * s) {
        cout << 50 << endl;
        return 0;
    }
    cout << int(50 + 10 * (n * x.at(m - 1) - s) / sqrt(var_n2)) << endl;
}