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


int main()
{
    long long n, m, total = 0, sqn2 = 0;
    cin >> n;
    vector<int> v(n);
    for (int i = 0; i < n; i++){
        cin >> v[i];
        total += v[i];
    }
    cin >> m;
    for (int i = 0; i <n; i++) sqn2 += (total - v[i]*n)*(total - v[i]*n);
    sqn2 = pow(sqn2, 0.5);
    cout << fixed << setprecision(0);
    long double ans = ((v[m-1]*n - total)*10*pow(n, 0.5)/sqn2 + 50);
    if (sqn2 == 0) cout << 50 << endl;
    else if (ans > 0) cout << floor(ans) << endl;
    else cout << ceil(ans) << endl;
}