n,m,C=map(int,input().split())
a=list(map(int,input().split()))
b=list(map(int,input().split()))
a.sort()
b.sort()
p=0
for i in range(n):
  ok=m
  ng=-1
  while ok-ng>1:
    mid=(ok+ng)//2
    if b[mid]*a[i]>C:
      ok=mid
    else:
      ng=mid
  p+=m-ok
print(p/(n*m))