#include <bits/stdc++.h>

using ll = long long;

using namespace std;

int main()
{
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    ll n;
    double sum{ 0.0 }, sum2{ 0.0 };

    cin >> n;
    vector<int> v(n);

    for (int i = 0; i < n; ++i)
    {
        cin >> v[i];
        sum += v[i];
    }
    cin.ignore();

    double avg = sum / n;
    for (int i = 0; i < n; ++i)
        sum2 += (avg-v[i]) * (avg - v[i]);

    long m;
    cin >> m;
    ll res{50};
    if (sum2)
        res += (v[m - 1] - sum / n) * 10 / sqrt(sum2 / n);

    printf("%lld\n", res);

    return 0;
}