import math

n = int(input())
x = list(map(int, input().split()))
m = int(input()) - 1  # Convert to 0-based index

x_m = x[m]
sum_x = sum(x)
mean = sum_x / n

sum_squares = 0.0
for xi in x:
    diff = xi - mean
    sum_squares += diff * diff

variance = sum_squares / n
std_dev = math.sqrt(variance)

if std_dev == 0:
    print(50)
else:
    temp = 10 * (x_m - mean) / std_dev
    if x_m > mean:
        ans_float = 50 + temp
    else:
        ans_float = 50 - temp
    print(int(ans_float))