#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);
    }
    if(S == 0){
        cout << 50 << endl;
        return 0;
    }
    if(N * x[M - 1] >= Ave){
        auto f = [&](ll v){
            ll d = (N * x[M - 1] - Ave) * 10;
            d *= d * N;
            return (v * v * S <= d);
        };
        ll ok = 0, ng = 100;
        while(ok + 1 < ng){
            ll m = (ok + ng) / 2;
            if(f(m))ok = m;
            else ng = m;
        }
        cout << 50 + ok << endl;
    }else{
        auto f = [&](ll v){
            ll d = (N * x[M - 1] - Ave) * 10;
            d *= d * N;
            return (v * v * S >= d);
        };
        ll ng = 0, ok = 100;
        while(ng + 1 < ok){
            ll m = (ok + ng) / 2;
            if(f(m))ok = m;
            else ng = m;
        }
        cout << 50 - ok << endl;
    }
}