#include <bits/stdc++.h>
using namespace std;
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n) FOR(i,0,n)
#define ALL(v) begin(v),end(v)
template<typename A, typename B> inline bool chmax(A & a, const B & b) { if (a < b) { a = b; return true; } return false; }
template<typename A, typename B> inline bool chmin(A & a, const B & b) { if (a > b) { a = b; return true; } return false; }
using ll = long long;
using pii = pair<int, int>;
constexpr ll INF = 1ll<<30;
constexpr ll longINF = 1ll<<60;
constexpr ll MOD = 1000000007;
constexpr bool debug = false;
//---------------------------------//

int main() {
	int N, M;
	ll C;
	cin >> N >> M >> C;
	vector<int> A(N), B(M);
	REP(i, N) scanf("%d", &A[i]);
	REP(i, M) scanf("%d", &B[i]);
	sort(ALL(A));
	sort(ALL(B));
	
	ll pat = 0;
	int r = M - 1;
	REP(i, N) {
		while (r >= 0 && B[r] > C / A[i]) --r;
		pat += M - r - 1;
	}
	// B[i] > C / A[i]
	double ans = pat / ((double)N * M);
	printf("%.16f\n", ans);
}