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

int main(){
    int N, M;
    cin >> N;
    vector<ll> x(N);
    for(int i = 0; i < N; i++)cin >> x[i];
    cin >> M;
    ll Ave = accumulate(x.begin(), x.end(), 0ll), S = 0;
    for(int i = 0; i < N; i++){
        S += (N * x[i] - Ave) * (N * x[i] - Ave);
    }
    //S = sqrt(S / (N * N * N));
    long double D = (N * x[M - 1] - Ave) * 10 / (N * sqrt(S / (N * N * N)));
    cout << int(floor(D + 50)) << endl;
}