#include<bits/stdc++.h>

using namespace std;
using ll = long long;


int main()
{
	int N, M; cin >> N >> M; ll C; cin >> C;
	vector<ll> A(N), B(M);
	for(int i = 0; i < N; i++)cin >> A[i];
	for(int i = 0; i < M; i++)cin >> B[i];
	sort(B.begin(), B.end());
	double prob = 0;

	for(int i = 0; i < N; i++) {
		ll mb = C / A[i];
		prob += (B.end() - upper_bound(B.begin(), B.end(), mb)) / (double)M;
	}
	
	cout << fixed <<setprecision(12) << prob / N << endl; 
	

}