using System;

namespace ExpandNumberForum
{
  class Program
  {
    public static void Main(string[] args)
    {
        string s = string.Empty;
        int b, c, n;

        Console.WriteLine("Input number: ");
        if (!int.TryParse(Console.ReadLine(), out n)) return;

        while ((n % 2) == 0)
        {
            n = n / 2;
            s += "2*";
        }
        b = 3; c = (int)Math.Sqrt(n) + 1;
        while (b < c)
        {
            if ((n % b) == 0)
            {
                if (n / b * b - n == 0)
                {
                    s += b.ToString() + "*";
                    n = n / b;
                    c = (int)Math.Sqrt(n) + 1;
                }
                else
                    b += 2;
            }
            else
                b += 2;
        }
        s += n.ToString();
        Console.WriteLine(s);

        Console.Write("Press any key to continue . . . ");
        Console.ReadKey(true);
    }
}
}

r = a - a / b * b