import std;

void main() {
    int N;
    readf("%d\n", N);

    auto x = readln.chomp.split.to!(real[]);

    int M;
    readf("%d\n", M);

    --M;

    real S = x.sum, A = S / N;

    real D = 0.0;
    foreach (u; x) {
        D += (u - A) ^^ 2;
    }
    D /= N;

    real Q = D.sqrt;
    real T = 50.0 + (x[M] - A) * 10.0 / Q;

    int res = T.floor.to!int;
    res.writeln;
}