#include <iostream>
#include <cmath>
#include <iomanip>


int main(void){
  using namespace std;
  int n=0, m=0,x[100000];
  double ave=0.0,var=0.0,st=0.0,ans=0.0;

  cin >> n;
  for(int i=0;i<n;i++){
    cin >> x[i];
    ave += (double)x[i];
  }
  cin >> m;

  ave = ave/(double)n;
  for(int i=0;i<n;i++) var += ((double)x[i]-ave)*((double)x[i]-ave);
  st=sqrt(var/n);  
  if(x[m-1] >= ave) ans = 50 + (x[m-1]-ave)*10.0/st;
  else ans = 50 + (x[m-1]-ave)*10.0/st;

  //cout << ave << endl;
  //cout << x[m-1] <<endl;
  //cout << ans <<endl;
  cout << fixed << setprecision(0) << floor(ans) << endl;
  
  return 0;    

}