#define _USE_MATH_DEFINES
#include  <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
#include <string>
#include<math.h>
#include<iomanip>
#include<stdio.h>
#include <stdlib.h>
#include<stdio.h>
#include <queue>
#include<map>
#include <fstream>
#include <sstream>
#include<set>
#include<stack>
#include<time.h>
#include <sys/types.h>
#include <tuple>
#include <numeric>
#include <iterator>

typedef long long int ll;
//#include<bits/stdc++.h>


using namespace std;

ll gcd(ll a, ll b) {
    if (a < b)swap(a, b);
    if (b == 0)return a;
    return gcd(b, a % b);
}
int lcm(int a, int b) {
    return a * b / gcd(a, b);
}







int main()
{

    int Q;
    Q = 1;
    //cin >> Q;   

    while (Q--) {
        int n;
        cin >> n;
        vector<double>p(n);
        for (int i= 0; i < n; i++) {
            cin >> p[i];
        }
        int m;
        cin >> m;
        double x = 0;
        for (int i = 0; i < n; i++)x += p[i];
        x /= (double)n;
        double d = 0;
        for (int i = 0; i < n; i++)d += (p[i] - x) * (p[i] - x);
        d /= (double)n;
        d = sqrt(d);
        if (abs(d) < 1e-12) {
            cout << 50 << endl;
            return 0;
        }
        m--;
        ll he = 10 * ( p[m]-x) / d;
        
        cout << 50 + he;
       
      
    }
        return 0;
}