#include <iostream>
#include <vector>
#include <cmath>
using namespace std;
const double EPS = 0.0001;
int main(){
	int N;
	cin >> N;
	vector<int> x(N);
	for (int i = 0; i < N; i++){
		cin >> x[i];
	}
	int M;
	cin >> M;
	M--;
	double m = 0;
	for (int i = 0; i < N; i++){
		m += x[i];
	}
	m /= N;
	double v = 0;
	for (int i = 0; i < N; i++){
		v += pow(x[i] - m, 2);
	}
	v /= N;
	double s = sqrt(v);
	if (s < EPS){
		cout << 50 << endl;
	} else {
		cout << (int) (50 + (x[M] - m) / s * 10) << endl;
	};
}