#include <iostream>
#include <string>
#include <algorithm>
#include <vector>
#include <iomanip>
#include <cmath>
#include <stdio.h>
#include <queue>
#include <deque>
#include <cstdio>
#include <set>
#include <map>
#include <bitset>
#include <stack>
#include <cctype>
using namespace std;
long double x[100010];
int main() {	
	long double n;
	int m;
	cin >> n;
	long double sum = 0;
	for (int i = 0; i < n; i++) {
		cin >> x[i];
		sum += x[i];
	}
	sum /= n;
	long double sum1 = 0;
	for (int i = 0; i < n; i++) {
		sum1 += (sum - x[i]) * (sum - x[i]);
	}
	sum1 /= n;
	sum1 = sqrt(sum1);
	cin >> m;
	long double ans = abs(x[m - 1] - sum) * 10 / sum1;
	if (x[m - 1] > sum) {
		cout << floor(50 + ans) << endl;
	}
	else {
		cout << floor(50 - ans) << endl;
	}
	return 0;
}