SumPrimeNonPrime 2

using System;

namespace SumPrimeNonPrime
{
    class SumPrimeNonPrime
    {
        static void Main()
        {

            string input = Console.ReadLine();
            int N = int.Parse(input);
            int nativeSum = 0;
            int nonNativeSum = 0;

            while (input != "stop")
            {
                bool IsPrime(int N)
                {
                    if (N <= 1) return false;
                    if (N == 2) return true;
                    if (N % 2 == 0) return false;

                    var boundary = (int)Math.Floor(Math.Sqrt(N));

                    for (int i = 3; i <= boundary; i += 2)
                        if (N % i == 0)
                            return false;

                    return true;
                }

                if (IsPrime(N))
                {
                    nativeSum += N;
                }
                else
                {
                    nonNativeSum += N;
                }

                input = Console.ReadLine();
                if (input == "stop")
                {
                    break;
                }
                else
                {
                    N = int.Parse(input);
                }
                if (N < 0)
                {
                    Console.WriteLine("Number is negative.");
                    input = Console.ReadLine();
                    N = int.Parse(input);
                }
            }
            Console.WriteLine($"Sum of all prime numbers is: {nativeSum}");
            Console.WriteLine($"Sum of all non prime numbers is: {nonNativeSum}");
        }
    }
}