import math

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

# Check if all elements are equal
first = x[0]
all_equal = True
for xi in x:
    if xi != first:
        all_equal = False
        break

if all_equal:
    print(50)
else:
    # Compute average
    avg = sum(x) / n
    # Compute sum of squared differences
    sum_sq = 0.0
    for xi in x:
        diff = xi - avg
        sum_sq += diff * diff
    variance = sum_sq / n
    std_dev = math.sqrt(variance)
    
    # Get x_m
    x_m = x[m-1]
    diff_x = x_m - avg
    
    # Compute t
    t = (10 * diff_x) / std_dev
    
    if diff_x > 0:
        deviation = 50 + t
    else:
        deviation = 50 - t
    
    # Floor the deviation and print as integer
    result = math.floor(deviation)
    print(int(result))