#include<stdio.h>
double root(double n)
{
	if (n == 0)
		return 0;
	double min, mid, max;
	min = 0;
	max = 1;
	while (max * max < n)
		max *= 2;
	for (int i = 0; i < 100; i++)
	{
		mid = (max + min) / 2;
		if (mid * mid < n)
			min = mid;
		else
			max = mid;
	}
	return max;
}
int main()
{
	int n;
	scanf("%d", &n);
	int i;
	double x[100005];
	for (i = 0; i < n; i++)
		scanf("%lf", &x[i]);
	int m;
	scanf("%d", &m);
	m--;
	double ave = 0;
	for (i = 0; i < n; i++)
		ave += x[i];
	ave /= n;
	double stdev = 0;
	for (i = 0; i < n; i++)
		stdev += (x[i] - ave) * (x[i] - ave);
	stdev /= n;
	stdev = root(stdev);
	if (stdev == 0)
		printf("50\n");
	else
		printf("%d\n", (int)((x[m] - ave) * 10 / stdev + 50));
	return 0;
}