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

int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  ll n, m, c;
  cin >> n >> m >> c;
  vector<ll> a(n), b(m);
  for (ll i = 0; i < n; i++) {
    cin >> a[i];
  }
  for (ll i = 0; i < m; i++) {
    cin >> b[i];
  }
  sort(begin(a), end(a));
  sort(begin(b), end(b));
  ll pos = n - 1, cnt = 0;
  for (ll i = 0; i < n; i++) {
    while (pos >= 0) {
      if (a[i] * b[pos] <= c) {
        break;
      } else {
        pos--;
      }
    }
    cnt += n - 1 - pos;
  }
  cout << setprecision(10);
  cout << (double)cnt / (n * m) << '\n';
}