// g++ A.cpp -std=c++14 -I . && ./a.out
#include <bits/stdc++.h>
using namespace std;
// #include <atcoder/all>
// using namespace atcoder;
// using mint = modint1000000007;
// using mint = modint998244353;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define rep2(i, a, b) for (int i = a; i < (int)(b); i++)
#define rrep(i, a, b) for (int i = a; i > (int)(b); i--)
#define all(v) v.begin(), v.end()
using ll = long long;
const ll INF = 1e18;
// 変数定義
ll N, M, C, Q, a, b, c, d, x, y, t, T, total, cnt, ans;
int main()
{
  cout << fixed << setprecision(10);
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  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));
  sort(all(B));
  int k = 0;
  rep(i, N)
  {
    rep2(j, k, M)
    {
      if (A[i] * B[j] > C)
      {
        ans += M - j;
        k = j;
        break;
      }
    }
  }
  cout << (double)ans / (N * M) << '\n';
  return 0;
}