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

int N, M;
double x[101010];

int main() {
  cin >> N;
  for (int i = 0; i < N; i++) cin >> x[i];
  cin >> M;
  double S = 0.0;
  for (int i = 0; i < N; i++) S += x[i];
  S /= double(N);
  double T = 0.0;
  for (int i = 0; i < N; i++) T += (S - x[i]) * (S - x[i]);
  T /= double(N);
  T = sqrt(T);
  if (abs(T) < 1e-12) {
    cout << 50 << endl;
    return 0;
  }
  double U = (x[M - 1] - S) * 10 / T;
  //cout << S << ' ' << T << ' ' << U << endl;
  double res = 50 + U;
  cout << ll(res) << endl;
}