import java.util.Scanner;


public class HelloWorld {
		public static void main (String[] args) {
			Scanner sc = new Scanner(System.in);
			int n = sc.nextInt();
			int[] xi = new int[n];
			int sum = 0;
			for (int i=0;i<n;i++) {
				xi[i] = sc.nextInt();
				sum += xi[i];
			}
			int m = sc.nextInt();
			sc.close();

			int average = sum / n;
			sum = 0;

			for (int i=0;i<n;i++) {
				sum = sum + (xi[i] - average) * (xi[i] - average);
			}
			double sqrt = Math.sqrt(sum / n);

			double ans = (xi[m - 1] - average) * 10 / sqrt;
			System.out.println(ans > 0 ? (int)(50 + ans) : (int)(50 - ans));


	}
}