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
from bisect import bisect_left as bl
num=0
for x in a:
  if c%x==0:
    idx=br(b,c//x)
    num+=m-idx
  else:
    idx=bl(b,(c+x-1)//x)
    num+=m-idx
print(num/(n*m))