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

using namespace std;
using LL = long long;

int main(){
	LL N, M;
	cin >> N >> M;
	LL C;
	cin >> C;
	vector<LL> A(N), B(M);
	rep(i,N) cin >> A[i];
	rep(i,M) cin >> B[i];
	double ans=0;
	rep(i,N){
		rep(j,M) if(A[i]*B[j]>C) ans++;
	}
	cout << setprecision(10) << ans/(N*M) << endl;

	return 0;
}