Nine-Digit Magic Numbers

using System;
using System.Collections.Generic;
using System.Linq;

class Program
{
    static void Main(string[] args)
    {
        // input
        int sum = int.Parse(Console.ReadLine());
        int diff = int.Parse(Console.ReadLine());

        // declarations
        IEnumerable<string> combinations = CombinationsWithRepition("1234567", 3);
        List<string> workCopy = combinations.ToList();
        List<int> numbers = workCopy.Select(int.Parse).ToList();
        List<string> result = new List<string>();

        // logic
        for (int i = 0; i < numbers.Count; i++)
        {
            int temp1 = numbers[i] + diff;
            if (numbers.Contains(temp1))
            {
                int temp2 = temp1 + diff;
                if (numbers.Contains(temp2))
                {

                    int tempSum = 0;
                    for (int l = 1; l <= 3; l++)
                    {
                        tempSum += GetDigit(numbers[i], l);
                        tempSum += GetDigit(temp1, l);
                        tempSum += GetDigit(temp2, l);
                    }

                    if (tempSum == sum)
                    {
                        string x = workCopy[i] + temp1.ToString() + temp2.ToString();
                        result.Add(x);
                    }
                }
            }
        }

        // printing
        if (result.Count > 0)
        {
            foreach (var item in result)
            {
                Console.WriteLine(item);
            }
        }
        else
        {
            Console.WriteLine("No");
        }
    }

    #region Create all combinations with repetitions 7 choose 3
    static String Convert(string symbols, int number, int totalLen)
    {
        var result = "";
        var len = symbols.Length;
        var nullSym = symbols[0];
        while (number > 0)
        {
            var index = number % len;
            number = number / len;
            result = symbols[index] + result;
        }
        return result.PadLeft(totalLen, nullSym);
    }

    static IEnumerable<String> CombinationsWithRepition(string symbols, int len)
    {
        for (var i = 0; i < Math.Pow(symbols.Length, len); i++)
            yield return Convert(symbols, i, len);
    }

    #endregion

    // GetDigit method - splits an int into digits
    static int GetDigit(int number, int digit)
    {
        return (number / (int)Math.Pow(10, digit - 1)) % 10;
    }
}