#include<bits/stdc++.h>
using namespace std;
int main() {
    long long N,M;
    cin >> N;
    vector<int> X;
    for(int i = 0; i < N; i++) cin >> X[i];
    cin >> M;
    double num = 0, sum = 0;
    for(int i = 0; i < N; i++) num += X[i];
    num /= N;
    for(int i = 0; i < N; i++) sum += (X[i] - num) * (X[i] - num);
    sum /= N;
    sum = pow(sum, 0.5);
    if (sum) {
        num = (X[M - 1] - num) * 10 / sum;
        cout << (long long) X[M - 1] + num << endl;
    } else {
        cout << 50 << endl;
    }
}