using System;

public class Program

{

    public static int Main()

    {

        Int64 i = 0, largestPrime = 0, N = 600851475143;//Int64.Parse(Console.ReadLine());

        Console.WriteLine("Finding the largest prime factor for number: {0}", N);

​

        Int64 squareRootNumber = Convert.ToInt64(System.Math.Ceiling(System.Math.Sqrt(N))), squareRootPrimeNumberTest = 0;

​

        for (Int64 p = 3; ; p += 2)

        {

            squareRootPrimeNumberTest = Convert.ToInt64(System.Math.Ceiling(System.Math.Sqrt(p)));

​

            for (i = 3; i <= squareRootPrimeNumberTest; i += 2)

            {

                if (p % i == 0)

                {

                    break;

                }

            }

            if (i > squareRootPrimeNumberTest)

            {

                if (N % p == 0)

                {

                    largestPrime = p;                        

                }

                if (p > squareRootNumber)

                {

                    Console.WriteLine("Largest prime factor: {0}", largestPrime);

                    return 0;

                }

            }

        }               

    }

}