Project Euler problem #3 solution.

using System;

public class problem
{
    static void Main()
    {
        long    a = 600851475143;
        int     b = Convert.ToInt32(Math.Sqrt(a));
        int c = 0;

        for (int i = b; i >= 2; i--)
        {
            if (a % i == 0)
                c = ChkPrime(i);
            if (c > 0)
                break;
        }

        Console.WriteLine(c);
    }

    static int ChkPrime(int c)
    {
        for (int i = 2; i < Math.Sqrt(c); i++)
            if (c % i == 0)
            {
                c = -1;
                break;
            }

        return c;
    }
}