LargestPrime

        static void Main(string[] args)
        {
            Stopwatch sw = new Stopwatch();
            sw.Start();
            long num = 600851475143;
            int start = 2;
          
            while (!isprime(num))
            {
                while (num % start != 0)
                {
                    start = getNextPrime(start);
                }
                
                num /= start;
            }
            sw.Stop();
            Console.WriteLine("calculted in {0} ",sw.Elapsed);
            Console.WriteLine(num);
        }
       
        static int getNextPrime(int i)
        {
            int x = ++i;
            while (!isprime(x))
            {
                x++;
            }
            return x;
        }
        static bool isprime(long n)
        {
            if (n < 2)
                return false;
            if (n == 2 || n == 3)
                return true;
            if (n % 2 == 0 || n % 3 == 0)
                return false;
            int max_divisor = (int)Math.Sqrt(n);
            int divisor = 5;
            while (divisor <= max_divisor)
            {
                if (n % divisor == 0 || n % (divisor + 2) == 0)
                    return false;
                divisor += 6;
            }
            return true;
        }