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

# Calculate sum of squares
sum_x_squared = sum(xi ** 2 for xi in x)
sum_sq_diff = sum_x_squared - n * (avg ** 2)
variance = sum_sq_diff / n
std = math.sqrt(variance)

x_m = x[m]
diff = x_m - avg

if std < 1e-9:
    deviation = 50
else:
    step3 = (diff * 10) / std
    if diff > 0:
        deviation = 50 + step3
    else:
        deviation = 50 - step3

result = math.floor(deviation)
print(result)