#include <bits/stdc++.h>

using namespace std;

int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  int64_t N, M, C;
  cin >> N >> M >> C;
  vector<int64_t> A(N), B(M);
  for (auto &&e : A) {
    cin >> e;
  }
  for (auto &&e : B) {
    cin >> e;
  }
  sort(A.begin(), A.end());
  sort(B.begin(), B.end());
  int64_t x = 0;
  for (int64_t a : A) {
    int lo = 0, hi = M;
    while (lo < hi) {
      int mi = (lo + hi) / 2;
      if (B[mi] * a > C) {
        hi = mi;
      } else {
        lo = mi + 1;
      }
    }
    x += M - hi;
  }
  cout << fixed << setprecision(10) << (long double)x / N / M << '\n';
  return 0;
}