#include <iostream>
#include <iomanip>
#include <vector>
#include <algorithm>
#include <cassert>
#define rep(i,n) for(int i=0; i<(int)(n); i++)

using namespace std;
using LL = long long;
const int Max_Num=1e5;
const LL Max_C=1e18;
const int Max_AB=1e9;

int main(){
	LL N, M, C;
	cin >> N >> M >> C;
	assert(N>=1 && N<=Max_Num);
	assert(M>=1 && M<=Max_Num);
	assert(C>=1 && C<=Max_C);
	vector<LL> A(N), B(M);
	rep(i,N){
		LL a;
		cin >> a;
		assert(a>=1 && a<=Max_AB);
		A[i]=a;
	}
	rep(i,M){
		LL b;
		cin >> b;
		assert(b>=1 && b<=Max_AB);
		B[i]=b;
	}
	sort(A.begin(),A.end(),greater<LL>());
	sort(B.begin(),B.end());
	double ans=0;
	int i=0, num=0;
	while(i<N){
		LL a=A[i];
		for(int j=num; j<M; j++){
			LL b=B[j];
			if(a*b>C){
				ans+=M-j;
				i++, num=j;
				break;
			}
			if(j==M-1) i=N;
		}
	}
	cout << setprecision(10) << ans/(N*M) << endl;

	return 0;
}