import math

n = int(input())
x = list(map(int, input().split()))
m = int(input())

x_m = x[m-1]

sum_x = sum(x)
avg = sum_x / n

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

var = sum_sq / n
std = math.sqrt(var) if var != 0 else 0.0

if std == 0:
    dev = 50
else:
    diff = x_m - avg
    step3 = (10 * diff) / std
    if x_m > avg:
        dev = 50 + step3
    else:
        dev = 50 - step3
    dev = int(dev)

print(dev)