#include <algorithm>
#include <cmath>
#include <functional>
#include <iomanip>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <tuple>
#include <vector>
using namespace std;
using ll = long long;
#define rep(i, j, n) for (int i = j; i < (n); ++i)
#define rrep(i, j, n) for (int i = (n)-1; j <= i; --i)

[[maybe_unused]] constexpr ll MOD = 998244353;
[[maybe_unused]] constexpr int INF = 0x3f3f3f3f;
[[maybe_unused]] constexpr ll INFL = 0x3f3f3f3f3f3f3f3fLL;

int main() {
  ll n, m, c;
  cin >> n >> m >> c;
  vector<ll> a(n), b(m);
  rep(i, 0, n) cin >> a[i];
  rep(i, 0, m) cin >> b[i];

  sort(a.rbegin(), a.rend());
  sort(b.begin(), b.end());

  int r = 0;
  ll cnt = 0;
  rep(i, 0, n) {
    while (r < m && a[i] * b[r] <= c) ++r;
    if (r == m) break;
    cnt += m - r;
  }
  cout << fixed << setprecision(10) << (double)cnt / (n * m) << '\n';

  return 0;
}