import math

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

xm = x[m - 1]

s = sum(x)
mu = s / n

sum_sq = 0.0
for xi in x:
    sum_sq += (xi - mu) ** 2

sum_sq = max(0.0, sum_sq)
var = sum_sq / n

epsilon = 1e-9
if var < epsilon:
    sigma = 0.0
else:
    sigma = math.sqrt(var)

if sigma == 0:
    print(50)
else:
    diff = xm - mu
    step3 = diff * 10 / sigma
    if diff > 0:
        step4 = 50 + step3
    elif diff < 0:
        step4 = 50 - step3
    else:
        step4 = 50
    score = int(step4)
    print(score)