#include <bits/stdc++.h>
#define PI 3.14159265359
using namespace std;
const int64_t MOD = 1e9 + 7;

int main() {
	double N;
	int M;
	cin >> N;

	vector<int> v(N);
	double avg = 0;
	for (int i = 0; i < N; i++) {
		cin >> v.at(i);
		avg += v.at(i);
	}
	avg /= N;
	//cout << "avg: " << avg << endl;

	cin >> M;

	double sqavg = 0.0;
	for (int i = 0; i < N; i++) {
		double sq = (abs(avg - v.at(i))) * (abs(avg - v.at(i)));
		sqavg += sq;
	}
	sqavg = sqrt(sqavg / N);
	//cout << "sqavg: " << sqavg << endl;

	double d = abs(avg - v.at(M - 1)) * 10 / sqavg;
	int ans;
	if (v.at(M - 1) > avg) {
		ans = (int)50 + d;
	}
	else {
		ans = (int)50 - d;
	}

	cout << ans << endl;
}