#include<bits/stdc++.h>
using namespace std;
#define int long long
#define ii pair <int, int>
#define app push_back
#define all(a) a.begin(), a.end()
#define bp __builtin_popcountll
#define ll long long
#define mp make_pair
#define f first
#define s second
#define Time (double)clock()/CLOCKS_PER_SEC
#define debug(x) std::cout << #x << ": " << x << '\n';
signed main() {
    #ifdef HOME
    freopen("input.txt", "r", stdin);
    #else
    #define endl '\n'
    ios_base::sync_with_stdio(0); cin.tie(0);
    #endif

    int n;
    cin >> n;
    vector <int> x(n);
    int sum = 0;
    for (int i = 0; i < n; ++i) {
        cin >> x[i];
        sum += x[i];
    }
    int m;
    cin >> m;

    double av = sum/n;
    
    double k = 0;
    for (int i = 0; i < n; ++i)
        k += (x[i] - av) * (x[i] - av);
    double r = sqrt(k/n);

    //debug(av);
    //debug(r);

    if (r == 0) {
        cout << 50 << endl;
    }   
    else {
        cout << signed((x[m-1]-av)*10/r+50) << endl;
    }   
}