#yuki1280
import bisect
n,m,c=map(int,input().split())
a=list(map(int,input().split()))
b=list(map(int,input().split()))
a.sort()
b.sort()
ok=0
for x in a:
 ok+=m-bisect.bisect_right(b,c//x)
print(ok/(n*m))