#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);
    }
    //long double D = (N * x[M - 1] - Ave) * 10 / (N * sqrt(S / (N * N * N)));
    //cout << int(floor(D + 50)) << endl;
    auto f = [&](ll v){
        ll d = (N * x[M - 1] - Ave) * 10;
        d *= d * N;
        return (v * v * S <= d);
    };
    ll ok = -10000, ng = 10000;
    while(ok + 1 < ng){
        ll m = (ok + ng) / 2;
        if(f(m))ok = m;
        else ng = m;
    }
    cout << 50 + ok << endl;
}