#include <iostream>
#include <algorithm>
#include <map>
#include <set>
#include <queue>
#include <stack>
#include <numeric>
#include <bitset>
#include <cmath>

static const int MOD = 1000000007;
using ll = long long;
using u32 = unsigned;
using u64 = unsigned long long;
using namespace std;

template<class T> constexpr T INF = ::numeric_limits<T>::max()/32*15+208;

int main() {
    int n, m;
    cin >> n;
    vector<int> v(n);
    for (auto &&i : v) scanf("%d", &i);
    cin >> m;
    ll S = accumulate(v.begin(),v.end(), 0LL);
    long double a = 1.0L*S/n;
    long double d = 0;
    for (int i = 0; i < n; ++i) {
        d += (v[i]-a)*(v[i]-a);
    }
    if(d < 1e-10) {
        puts("50");
        return 0;
    }
    d = sqrt(d/n);
    long double ans = trunc(10*(v[m-1]-a)/d+50);
    printf("%.0Lf\n", ans);
    return 0;
}