#include<iostream>
#include<map>
#include<vector>
#include <algorithm>
#include<math.h>
#include <iomanip>
#include<set>
#include <numeric>
using namespace std;


int main()
{
    double n, m, ave = 0, sq = 0;
    cin >> n;
    vector<int> v(n);
    for (int i = 0; i < n; i++){
        cin >> v[i];
        ave += v[i];
    }
    cin >> m;
    ave /= n;
    for (int i = 0; i <n; i++) sq += (ave - v[i])*(ave - v[i]);
    sq /= n;
    sq = pow(sq, 0.5);
    if (sq == 0) cout << 50 << endl;
    else cout << fixed << setprecision(0) << (v[m-1] - ave)*10/sq + 50 << endl;
}