from math import sqrt

n = int(input())
x = list(map(int, input().split()))
m = int(input()) - 1

average = sum(x) / n
a = [abs(i - average) ** 2 for i in x]
b = sqrt(sum(a) / n)
if b == 0:
    print(50)
    exit(0)
c = abs(average - x[m]) * 10 / b
if x[m] > average:
    print(int(50 + c))
else:
    print(int(50 - c))
# print("average =", average)
# print("b =", b)
# print("c =", c)