#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for(int(i) = 0; (i) < (n); (i)++)
#define FOR(i, m, n) for(int(i) = (m); (i) < (n); (i)++)
#define All(v) (v).begin(), (v).end()
#define pb push_back
#define MP(a, b) make_pair((a), (b))
template <class T> vector<T> make_vec(size_t a, T val) {
    return vector<T>(a, val);
}
template <class... Ts> auto make_vec(size_t a, Ts... ts) {
    return vector<decltype(make_vec(ts...))>(a, make_vec(ts...));
}
using ll = long long;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
using Graph = vector<vector<int>>;
template <typename T> struct edge {
    int to;
    T cost;
    edge(int t, T c) : to(t), cost(c) {}
};
template <typename T> using WGraph = vector<vector<edge<T>>>;
const int INF = 1 << 30;
const ll LINF = 1LL << 60;
const int MOD = 1e9 + 7;

int main() {
    int N, M;
    ll C;
    cin >> N >> M >> C;
    vector<ll> A(N), B(M);
    rep(i, N) cin >> A[i];
    rep(i, M) cin >> B[i];
    sort(All(A));
    reverse(All(A));
    sort(All(B));
    ll cur = 0;
    ll Bwin = 0;
    for(int i = 0; i < N; i++) {
        while(cur < M && A[i] * B[cur] <= C)
            cur++;
        Bwin += cur;
    }
    cout << fixed << setprecision(10)
         << (double)(N * M - Bwin) / (double)(N * M) << endl;
}