#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define rep(i,n) for(ll i=0;i<n;i++)

void solve() {
  int n;
  cin>>n;
  int x[n];
  int sum=0;
  rep(i,n){
    cin>>x[i];
    sum+=x[i];
  }
  double ave=(double)sum/(double)n;
  //cout<<ave<<endl;
  double sqsum=0;
  rep(i,n){
    sqsum+=(x[i]-ave)*(x[i]-ave);
  }
  double rsqsum= sqrt((double)sqsum/(double)n);
  //cout<<rsqsum<<endl;
  //cout<<sqsum<<endl;
  
    
  int m;
  cin>>m;
  m--;
  if(x[m]==ave){
    cout<<50<<endl;
    return;
  }

  if(x[m]>ave){
    cout<<fixed<<setprecision(0)<<50+abs(x[m]-ave)*10/rsqsum<<endl;
    return;
  }
  if(x[m]<ave){
    cout<<fixed<<setprecision(0)<<50-abs(x[m]-ave)*10/rsqsum<<endl;
    return;
  }  
  
  
}

signed main(){
  solve();
}