import math

N = int(input())
x_list = list(map(int, input().split()))
M = int(input())

sum_x = sum(x_list)
n = N
sum_sq_diff_num = 0

for x in x_list:
    term = x * n - sum_x
    sum_sq_diff_num += term ** 2

if sum_sq_diff_num == 0:
    print(50)
else:
    sigma_squared = sum_sq_diff_num / (n ** 3)
    sigma = math.sqrt(sigma_squared)
    x_m = x_list[M - 1]
    difference_numerator = x_m * n - sum_x
    t = (difference_numerator * 10) / (n * sigma)
    if difference_numerator > 0:
        dev = 50 + t
    else:
        dev = 50 - t
    dev_int = int(dev)
    print(dev_int)