n,m,c=map(int,input().split())
a=list(map(int,input().split()))
b=list(map(int,input().split()))
b.sort()
from bisect import bisect_right as br
num=0
for x in a:
  t=c/x
  idx=br(b,t)
  num+=m-idx
print(num/(n*m))