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

e = sum(X)/n
import math
t = 0
for x in X:
    t += (x-e)**2
t /= n
t = math.sqrt(t)
if abs(t) <= 10**(-6):
    print(50)
    exit()

d = (X[m]-e)*10
d /= t
print(int(50+d))