using System;
using static System.Console;
using System.Linq;
using System.Collections.Generic;

class Program
{
    static int NN => int.Parse(ReadLine());
    static long[] NList => ReadLine().Split().Select(long.Parse).ToArray();
    public static void Main()
    {
        Solve();
    }
    static void Solve()
    {
        var p = NList;
        var (n, m, c) = (p[0], p[1], p[2]);
        var a = NList;
        var b = NList;
        Array.Sort(a);
        Array.Sort(b);
        var ans = 0.0;
        var bi = m - 1;
        for (var i = 0; i < a.Length; ++i)
        {
            while (bi >= 0 && a[i] * b[bi] > c) --bi;
            ans += m - bi - 1;
        }
        WriteLine(ans / n / m);
    }
}