import math

n = int(input())
x = list(map(int, input().split()))
m = int(input())

sum_x = sum(x)
mu = sum_x / n

sum_sq_diff = 0.0
for xi in x:
    sum_sq_diff += (xi - mu) ** 2

variance = sum_sq_diff / n
sigma = math.sqrt(variance)

x_m = x[m - 1]

if sigma == 0:
    print(50)
else:
    temp = (x_m - mu) * 10 / sigma
    if x_m > mu:
        dev = 50 + temp
    else:
        dev = 50 - temp
    # Truncate towards zero
    print(int(dev))