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

int main(){
    int N, M;
    cin >> N;
    vector<int> x(N);
    long double Ave = 0, S = 0;
    for(int i = 0; i < N; i++){
        cin >> x[i];
        Ave += x[i];
    }
    cin >> M;
    Ave /= N;
    for(int i = 0; i < N; i++){
        S += (x[i] - Ave) * (x[i] - Ave);
    }
    if(S == 0){
        cout << 50 << endl;
        return 0;
    }
    S = sqrt(S / N);
    long double D = (x[M - 1] - Ave) * 10 / S;
    long double ans = 50 + D;
    cout << (ans < 0 ? -floor(abs(ans)) : floor(ans)) << endl;
}