Matematické Fórum

class Program
{
    static void Main()
    {
        bool should_end = false;
        ulong input;

        while (!should_end)
        {
            System.Console.Write("Input:\t");

            if (!ulong.TryParse(System.Console.ReadLine(), out input))
                continue;

            input = GetPrimeFactorital(input);

            ulong i = input;
            int count = 0;
            while (true)
            {
                if (!IsPrime(++i))
                    continue;

                if (!IsPrime(i - input))
                    break;

                count++;
                System.Console.WriteLine("\t{0}:\t{1} is prime", i, i - input);
            }

            System.Console.WriteLine("Number of primes:\t{0}", count);
            System.Console.WriteLine("First composite:\t{0}", i - input);
            System.Console.WriteLine();
        }
    }

    static ulong GetPrimeFactorital(ulong value)
    {
        ulong temp = 1;

        
        for (ulong i = 1; i <= value; i++)
            if (IsPrime(i))
                temp = checked(temp * i);
          
        return temp;
    }

    static bool IsPrime(ulong value)
    {
        if (primes.Contains(value))
            return true;

        double limit = System.Math.Sqrt(value) + 1;
        for (ulong i = 2; i < limit; i++)
            if (value % i == 0)
                return false;

        return true;
    }

    static Program()
    {
        primes = new System.Collections.Generic.HashSet<ulong>();

        primes.Add(1);
        primes.Add(2);
        primes.Add(3);
        primes.Add(5);
    }

    static System.Collections.Generic.HashSet<ulong> primes;
}