#include "bits/stdc++.h"
using namespace std;


int main() {
	long long N, M; cin >> N >> M;
	long long C; cin >> C;
	vector<long long>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());
	long long cnt = 0;
	for (int i = 0; i < N; i++) {
		long long now = C / a[i];
		cnt += (M - (upper_bound(b.begin(), b.end(), now) - b.begin()));
	}
	long  double ans = (long  double)(cnt) / (long double)(N*M);
	cout << fixed << setprecision(15) << ans << endl;
}