#include<iostream>
#include<vector>
#include<algorithm>
#include<cstring>
#include<cassert>
#include<cmath>
#include<ctime>
#include<iomanip>
#include<numeric>
#include<stack>
#include<queue>
#include<map>
#include<unordered_map>
#include<set>
#include<unordered_set>
#include<bitset>
#include<random>
using namespace std;
int N,M,A[1 << 17],B[1 << 17];
long long C;
void solve()
{
	cin >> N >> M >> C;
	for(int i = 0;i < N;i++) cin >> A[i];
	for(int i = 0;i < M;i++) cin >> B[i];
	sort(A,A+N,greater<int>());
	sort(B,B+M);
	int bi = 0;
	long long num = 0;
	for(int i = 0;i < N;i++)
	{
		while(bi < M && (long long)A[i]*B[bi] <= C) bi++;
		num += M-bi;
	}
	long long den = (long long)N*M;
	cout << fixed << setprecision(16);
	cout << (double)num/den << endl;
}
int main()
{
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	int tt = 1;
	//cin >> tt;
	while(tt--) solve();
}