#include <algorithm>
#include <array>
#include <cmath>
#include <cstdio>
#include <deque>
#include <iomanip>
#include <iostream>
#include <numeric>
#include <optional>
#include <queue>
#include <set>
#include <stack>
#include <unordered_map>
#include <unordered_set>
#include <vector>

using namespace std;

int main() {
    int32_t n;
    cin >> n;
    vector<int32_t> xs(n);
    for (auto &&x : xs) {
        cin >> x;
    }
    int32_t m;
    cin >> m;
    double ave = accumulate(xs.begin(), xs.end(), int32_t(0)) / double(n);
    vector<double> ys(n);
    for (auto i = 0; i < n; ++i) {
        double t = xs[i] - ave;
        ys[i] = t * t;
    }
    double t = sqrt(accumulate(ys.begin(), ys.end(), double(0)) / n);
    int32_t ans;
    if (t < 1e-12)
        ans = 50;
    else
        ans = 50 + (xs[m - 1] - ave) * 10 / t;
    cout << ans << endl;
    return 0;
}