#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
using ll=long long;
#define rep(i,a,b) for(it i=(it)(a);i<=(it)b;i++)
#define nrep(i,a,b) for(it i=(it)(a);i>=(it)b;i--)
#define all(v) v.begin(), v.end()
#define rall(v) v.rbegin(), v.rend()
#define moda 998244353LL
#define modb 1000000007LL
#define dai 2500000000000000000LL
#define tyu 2500000000
#define en 3.14159265358979
#define sho -dai
#define eps 1e-14
#define yn(x) cout<<(x ? "Yes":"No")<<endl;
#define YN(x) cout<<(x ? "YES":"NO")<<endl;
using it=long long;
using un=unsigned long long;
using db=long double;
using st=string;
using ch=char;
using bo=bool;
using P=pair<it,it>;
using mint=modint;
using mints=modint998244353;
using vi=vector<it>;
using vd=vector<db>;
using vs=vector<st>;
using vc=vector<ch>;
using vb=vector<bo>;
using vp=vector<P>;
using vm=vector<mint>;
using sp=set<P>;
using ss=set<st>;
using sc=set<ch>;
using si=set<it>;
using svi=set<vi>;
using vvi=vector<vi>;
using vvd=vector<vd>;
using vvs=vector<vs>;
using vvb=vector<vb>;
using vvc=vector<vc>;
using vvp=vector<vp>;
using vvm=vector<vm>;
using vsi=vector<si>;
using vsp=vector<sp>;
using vvsi=vector<vsi>;
using vvsp=vector<vsp>;
using vvvi=vector<vvi>;
using vvvd=vector<vvd>;
using vvvvi=vector<vvvi>;

int main(){
  it n;db hyo=0,hei=0;cin>>n;
  vd x(n);
  rep(i,0,n-1){
    cin>>x[i];
    hei+=x[i];
  }
  hei/=n;
  rep(i,0,n-1)hyo+=(x[i]-hei)*(x[i]-hei);
  hyo=sqrt(hyo/n);
  it m;cin>>m;
  db ans=(x[m-1]-hei)*10/hyo+50;
  cout<<fixed<<setprecision(0);
  cout<<(ans>0?floor(ans):ceil(ans))<<endl;
}