import math

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

x_m = x[m]
avg = sum(x) / n
sum_sq_diff = sum((xi - avg) ** 2 for xi in x)
variance = sum_sq_diff / n
sd = math.sqrt(variance)

if sd == 0:
    print(50)
else:
    t = (x_m - avg) * 10 / sd
    if x_m > avg:
        dev = 50 + t
    else:
        dev = 50 - t
    print(int(dev))