import math

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

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

numerator = sum_x_sq * n - sum_x * sum_x

if numerator == 0:
    print(50)
else:
    avg = sum_x / n
    variance = numerator / (n_sq)
    std_dev = math.sqrt(variance)
    
    diff = student_score - avg
    T = (diff * 10) / std_dev
    
    if student_score > avg:
        dev = 50 + T
    else:
        dev = 50 - T
    
    # Truncate towards zero
    print(int(dev))