#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < (n); i++)
using namespace std;
typedef long long ll;

int main(){
    cin.tie(0);
    ios::sync_with_stdio(0);
    
    int N; cin >> N;
    vector<double> x(N);
    rep(i,N) cin >> x[i];
    int M; cin >> M; M--;

    double mean = accumulate(x.begin(), x.end(), 0) / double(N);
    double se = 0;
    rep(i,N) se += (x[i] - mean) * (x[i] - mean);
    se /= double(N);
    se = sqrt(se);
    if(se <= 1e-8) { cout << 50 << endl; return 0; }
    double ans = (x[M] - mean) * 10.0 / se + 50.0;
    cout << floor(ans) << endl;
}