ll a[1d5],b[1d5],z;
{
	ll @n,@m,@c;
	rd(a(n),b(m));
	sort(a,a+n);
	sort(b,b+m);
	ll i=0,j=m-1;
	while(i<n&&j>=0){
		if(a[i]*b[j]>c){
			z+=n-i;
			--j;
		}else{
			++i;
		}
	}
	wt(1.0*z/(n*m));
}