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()
ans=0
for i in a:
    tmp=C/i
    ans+=len(b)-bisect.bisect_right(b,tmp)
print(ans/len(a)/len(b))