import math

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

sum_x = sum(x)
avg = sum_x / n

sum_x_sq = sum(xi * xi for xi in x)
sum_sq_diff = sum_x_sq - (sum_x ** 2) / n

# Handle potential negative values due to floating point precision
if sum_sq_diff < 0:
    sum_sq_diff = 0.0

variance = sum_sq_diff / n
std_dev = math.sqrt(variance) if variance != 0 else 0.0

x_m = x[m]

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