Ex2 Tink Back

using System;

namespace ConsoleApp1
{
    class Program
    {
        public static void Main(string[] args)
        {
            ulong n, a, b;
            ulong deductibleAmount = 0;
            var input = Console.ReadLine();
            if (input == null) return;
            n = Convert.ToUInt64(input.Split(" ")[0]);
            a = Convert.ToUInt64(input.Split(" ")[1]);
            b = Convert.ToUInt64(input.Split(" ")[2]);
            ulong summaToN = (1 + n) * n / 2;
            for (ulong i = a; i <= n; i += a)
            {
                deductibleAmount += i;
            }

            for (ulong i = b; i <= n; i += b)
            {
                if (i % a != 0)
                {
                    deductibleAmount += i;
                }
            }

            Console.WriteLine(summaToN - deductibleAmount);
        }
    }
}