#include <bits/stdc++.h>
using namespace std;

using Int = int_fast64_t;
using Word = uint_fast64_t;
using VInt = vector<Int>;
using VVI = vector<VInt>;
using VWord = vector<Word>;
using VVW = vector<VWord>;
using VS = vector<string>;
using VVS = vector<VS>;
using VB = vector<bool>;
using VVB = vector<VB>;
using PII = pair<Int,Int>;
using PWW = pair<Word,Word>;
using VPII = vector<PII>;
using VPWW = vector<PWW>;

#define SZ(x) ((Int)(x).size())
#define UNIQUE(v) v.erase(unique(v.begin(), v.end()), v.end())
#define rep(i,n) for(Int i=0, i##_len=(n); i<i##_len; ++i)
#define reps(i,n) for(Int i=0, i##_len=(n); i<=i##_len; ++i)
#define Range(i,a,b) for(Int i=a; i<=(Int)(b); i++)
#define ALL(x) (x).begin(),(x).end()
#define RALL(x) (x).rbegin(),(x).rend()
#define pb push_back
#define eb emplace_back

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

  Int n, m;
  cin >> n;
  VInt x(n);
  rep(i,n) cin >> x[i];
  cin >> m;
  m--;

  if (count(ALL(x), x[0]) == n) {
    cout << "50\n";
    return 0;
  }

  double mean = accumulate(ALL(x),0.0) / n;
  double var = 0;
  rep(i,n) var += (x[i] - mean) * (x[i] - mean);
  var /= n;
  double sd = sqrt(var);
  cout << Int(10 * (x[m] - mean) / sd + 50);

  return 0;
}