#include <bits/stdc++.h>

using namespace std;

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int N;
  cin >> N;
  vector<int> X(N);
  for (int i = 0; i < N; i++) {
    cin >> X[i];
  }
  int M;
  cin >> M;
  M--;
  double ave = 0;
  for (int x : X) {
    ave += x;
  }
  ave /= N;
  double a = 0;
  for (int x : X) {
    a += (ave - x) * (ave - x);
  }
  a /= N;
  a = sqrt(a);
  if (a == 0) {
    cout << 50 << '\n';
  } else {
    cout << (int)((X[M] - ave) * 10 / a + 50) << '\n';
  }
  return 0;
}