#include <bits/stdc++.h>
#define rep(i,n) for(int i=(0);i<(n);i++)

using namespace std;

typedef long long ll;

template<class T> bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T> bool chmin(T &a, const T &b) { if (a>b) { a=b; return 1; } return 0; }

int main(){
	cin.tie(0);
	ios::sync_with_stdio(false);

	ll n, m, c;
	cin >> n >> m >> c;

	vector<ll> a(n), b(m);
	rep(i, n) cin >> a[i];
	rep(i, m) cin >> b[i];

	sort(a.begin(), a.end());
	sort(b.begin(), b.end());

	ll tmp = 0;
	rep(i, n){
		auto it = upper_bound(b.begin(), b.end(), c / a[i]);
		if(it == b.end()) continue;
		tmp += m - distance(b.begin(), it);
	}

	cout << fixed << setprecision(10);
	cout << (double) tmp / (n * m) << endl;
}