#yuki1134
from math import *
n=int(input())
l=list(map(int,input().split()))
m=int(input())-1
s=sum(l)/n
a=[(s-l[i])**2 for i in range(n)]
x=sqrt(sum(a)/n)
if x>0:
 print(int((l[m]-s)*10/x+50))
else:
 print(50)