#include <bits/stdc++.h>
using namespace std;
template<typename T1,typename T2> inline void chmin(T1 &a,T2 b){if(a>b) a=b;}
template<typename T1,typename T2> inline void chmax(T1 &a,T2 b){if(a<b) a=b;}
using Int = long long;
const char newl = '\n';


template<typename T> void drop(const T &x){cout<<x<<endl;exit(0);}

//INSERT ABOVE HERE
signed main(){
  cin.tie(0);
  ios::sync_with_stdio(0);

  Int n;
  cin>>n;
  vector<Int> xs(n);
  for(Int i=0;i<n;i++) cin>>xs[i];
  Int m;
  cin>>m;
  m--;

  if(xs==vector<Int>(n,xs[0])) drop(50);

  using D = long double;
  D ave=0;
  for(Int x:xs) ave+=x;
  ave/=n;

  D squ=0;
  for(Int x:xs) squ+=(x-ave)*(x-ave);
  squ/=n;

  cout<<Int(50+(xs[m]-ave)*10/sqrt(squ))<<newl;
  return 0;
}