import math

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

total = sum(x)
avg = total / n

squared_diffs = [(xi - avg) ** 2 for xi in x]
variance = sum(squared_diffs) / n
std = math.sqrt(variance)

if std == 0:
    dev = 50
else:
    x_m = x[m]
    diff = x_m - avg
    dev = 50 + (diff * 10) / std
    dev = int(dev)  # truncate decimal part

print(dev)