#include <bits/stdc++.h>
using namespace std;

int main() {
  long N, M, C;
  cin >> N >> M >> C;
  vector<int> A(N), B(M);
  for (int i = 0; i < N; i++) cin >> A.at(i);
  for (int i = 0; i < M; i++) cin >> B.at(i);

  sort(B.begin(), B.end());

  long cnt = 0;
  for (auto a : A) {
    int tmp = upper_bound(B.begin(), B.end(), (long double)C / a) - B.begin();
    cnt += M - tmp;
  }
  cout << fixed << setprecision(6) << (long double)cnt / N / M << "\n";
}