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

#define ll long long
#define rep(i, n) for (int i = 0; i < (n); i++)
#define P pair<int, int>

int main() {
  ll N, M, C;
  cin >> N >> M >> C;

  ll a[N], b[M];
  rep(i, N) cin >> a[i];
  rep(i, M) cin >> b[i];

  sort(a, a+N);
  sort(b, b+M);

  ll p = 0;
  // a[i] に対して a[i] * b[j] > C となる j がいくつあるかを lower_bound を用いて求める．

  rep(i, N) {
    ll t = lower_bound(b, b+M, C/a[i]+1) - b;
    p += M-t;
  }


  double ans = (double)p/(M*N);

  cout << fixed << setprecision(10) << ans << endl;


}