#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
#define REP(i, n) for (int i = 0; i < (n); ++i)
#define REPR(i, n) for (int i = n - 1; i >= 0; --i)
#define FOR(i, m, n) for (int i = m; i < n; ++i)
#define FORR(i, m, n) for (int i = m; i >= n; --i)
#define ALL(v) (v).begin(),(v).end()
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; }
const ll INF=1LL<<60;
const int inf=(1<<30)-1;
const int mod=1e9+7;
int dx[8]={1,0,-1,0,-1,-1,1,1};
int dy[8]={0,1,0,-1,-1,1,-1,1};
int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);
    double n;cin >> n;
    vector<double> x(n);
    double sum=0;
    REP(i,n){
        cin >> x[i];
        sum+=x[i];
    }
    double ave=sum/n;
    int m;cin >> m;
    double ssum=0;
    REP(i,n){
        ssum+=(x[i]-ave)*(x[i]-ave);
    }
    double s2=ssum/n;
    double s=sqrt(s2);
    if(s==0){
        cout << 50 << endl;
        return 0;
    }
    double res=50+(x[m-1]-ave)*10/s;
    if(res>=0){
        cout << floor(res) << endl;
    }
    else{
        cout << ceil(res) << endl;
    }
}