import java.util.*;

public class Main {
	public static void main (String[] args) {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int[] arr = new int[n];
		double sum = 0;
		for (int i = 0; i < n; i++) {
		    arr[i] = sc.nextInt();
		    sum += arr[i];
		}
		double mean = sum / n;
		double ssum = 0;
		for (int i = 0; i < n; i++) {
		    ssum += (arr[i] - mean) * (arr[i] - mean);
		}
		double sd = Math.sqrt(ssum / n);
		int m = sc.nextInt() - 1;
		int ans = (int)((arr[m] - mean) * 10 / sd + 50);
		System.out.println(ans);
	}
}