import math

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

mu = sum(x) / n

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

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

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