import math

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

sum_x = sum(x)
sum_x_sq = sum(xi * xi for xi in x)

x_m = x[m]

# Calculate sum_sq_diff using precise method
numerator = sum_x_sq * n - sum_x * sum_x
sum_sq_diff = numerator / n  # divided by n here

if sum_sq_diff == 0.0:
    print(50)
else:
    avg = sum_x / n
    variance = sum_sq_diff / n
    std_dev = math.sqrt(variance)
    
    difference = x_m - avg
    step3 = (difference * 10) / std_dev
    
    if x_m > avg:
        dev_score = 50.0 + step3
    else:
        dev_score = 50.0 - step3
    
    # Truncate towards zero
    result = math.trunc(dev_score)
    print(result)