#include <bits/stdc++.h>

using namespace std;

#define REP(i,n) for(int i=0; i<(int)(n); i++)

const double EPS = 1e-9;

int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);

  int n;
  cin >> n;

  vector<int> x(n);
  REP (i, n) cin >> x[i];

  int m;
  cin >> m;
  --m;

  double avg = 0.0;
  REP (i, n) avg += x[i];
  avg /= n;
  
  vector<double> diff(n);
  REP (i, n)
    diff[i] = (avg - x[i]) * (avg - x[i]);

  double avg2 = 0;
  REP (i, n) avg2 += diff[i];
  avg2 /= n;
  avg2 = sqrt(avg2);

  if (avg2 == 0)
    cout << 50 << endl;
  else {
    double ret = 50 + 10 * (x[m] - avg) / avg2;
    cout << int(ret + EPS) << endl;
  }
  return 0;
}