#include <bits/stdc++.h>
using namespace std;
#ifdef LOCAL
  #include "settings/debug.cpp"
  #define _GLIBCXX_DEBUG
#else
  #define Debug(...) void(0)
#endif
#define rep(i, n) for (int i = 0; i < (n); ++i)
using ll = long long;
using ull = unsigned long long;

int main() {
  int n;
  cin >> n;
  vector<int> a(n);
  rep(i, n) cin >> a[i];
  int m;
  cin >> m;
  --m;

  double avg = accumulate(a.begin(), a.end(), 0) / (double) n;
  double diff = 0;
  rep(i, n) diff += (a[i] - avg) * (a[i] - avg);
  diff = sqrt(diff / n);
  double ans = (a[m] - avg) / diff * 10 + 50;
  cout << (int) floor(ans) << endl;
  return 0;
}