import math

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

sum_x = sum(x)
sum_x_sq = sum(a * a for a in x)
avg = sum_x / n

numerator = sum_x_sq - (sum_x ** 2) / n
variance = numerator / n

if abs(variance) < 1e-9:
    std_dev = 0.0
else:
    std_dev = math.sqrt(variance)

x_m = x[m]

if std_dev == 0:
    print(50)
else:
    diff = x_m - avg
    temp = (diff * 10) / std_dev
    if x_m > avg:
        dev = 50 + temp
    else:
        dev = 50 - temp
    print(int(dev))