import math

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

sum_x = sum(x)
sum_x_squared = sum(xi ** 2 for xi in x)

mean = sum_x / n
sum_sq_diff = sum_x_squared - n * (mean ** 2)
variance = sum_sq_diff / n

std_dev = math.sqrt(variance)

if std_dev == 0:
    print(50)
else:
    x_m = x[m - 1]
    factor = 10 * (x_m - mean) / std_dev
    deviation = 50 + factor
    print(int(deviation))