#include <iostream>
#include <numeric>
#include <cmath>
#include <vector>

using ldouble = long double;
constexpr ldouble EPS = 1e-10;

void solve() {
    int n;
    std::cin >> n;

    std::vector<ldouble> xs(n);
    for (auto& x : xs) std::cin >> x;

    ldouble ave = std::accumulate(xs.begin(), xs.end(), 0.L) / n;
    ldouble sv = std::sqrt(std::accumulate(xs.begin(), xs.end(), 0.L,
                                           [&](ldouble acc, ldouble x) {
                                               return acc + (x - ave) * (x - ave);
                                           }) /
                           n);

    if (sv < EPS) {
        std::cout << 0 << "\n";
        return;
    }

    int m;
    std::cin >> m;
    ldouble x = xs[--m];

    ldouble ans = (x - ave) * 10 / sv + 50;
    std::cout << (int)ans << "\n";
}

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

    solve();

    return 0;
}