#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <functional>
#include <cmath>
#include <iomanip>
#include <stack>
#include <queue>
#include <numeric>
#include <map>
#include <unordered_map>
#include <set>
#include <fstream>
#include <chrono>
#include <random>
#include <bitset>
//#include <atcoder/all>
#define rep(i,n) for(int i=0;i<(n);i++)
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define sz(x) ((int)(x).size())
#define pb push_back
using ll = long long;
using namespace std;
template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return 1; } return 0; }
ll gcd(ll a, ll b) {return b?gcd(b,a%b):a;}
ll lcm(ll a, ll b) {return a/gcd(a,b)*b;}

int main(){
  ll N; cin >> N;
  vector<ll> A(N); rep(i,N) cin >> A[i];
  int M; cin >> M; M--;
  ll sum = accumulate(all(A),0LL);
  if(sum==A[M]*N){
    cout << 50 << endl;
    return 0;
  }
  else{
    ll a=0;
    rep(i,N) a+=(sum-N*A[i])*(sum-N*A[i]);
    ll s = 50+10*(N*A[M]-sum)/sqrt(a/N);
    cout << s << endl;
  }
  return 0;
}