import math
N=int(input())
x=[int(x) for x in input().split()]
M=int(input())-1
e=sum(x)/N
y=[(v-e)**2 for v in x]
a=math.sqrt(sum(y)/N)
ans=(e-x[M])*10/a
if ans>e:
    ans=50+ans
else:
    ans=50-ans
print(int(ans))