#include <bits/stdc++.h>
 
using namespace std;
 
typedef long long ll;
typedef vector<ll> vll;
typedef vector<vll> vvll;
typedef pair<ll,ll> P;
typedef string str;
typedef vector<P> vp;
typedef vector<string> vs;
typedef vector<bool> vb;

const ll mod=1e9+7;
const ll inf=1e16;

#define rep(i,m,n) for(ll i=m;i<n;i++)
#define repr(i,m,n) for(ll i=m-1;i>=n;i--)
#define fi first
#define se second
#define chmax(x,y) x=max(x,y)
#define chmin(x,y) x=min(x,y)
#define eb(x) emplace_back(x)
#define pb(x) pop_back(x)
#define all(x) x.begin(),x.end()
#define allr(x) x.rbegin(),x.rend()
#define sum(x) accumulate(all(x),0)
#define pc(x) __builtin_popcount(x)
#define gll greater<ll>()

const ll m=200005;
vll imos(2*m,0);

void solve(){
  ll n;
  cin >> n;
  vll x(n);
  rep(i,0,n) cin >> x[i];
  ll m;
  cin >> m;
  m--;
  double ave=sum(x)/n;
  double st=0;
  rep(i,0,n) st+=(x[i]-ave)*(x[i]-ave);
  st=sqrt(st/n);
  double ans=(x[m]-ave)*10/st;
  ans+=50;
 cout << floor(ans+0.001) << endl;
  
  
  
}

int main(){
   cin.tie(nullptr);
  ios::sync_with_stdio(false);
  solve();
}