using System;

using System.Linq;

using System.Collections.Generic;

​

public class HelloWorld

{

    static void Main(string[] args)

    {

        double sumOfAllDigits = 0; // Final result

​

        long limit = 1000000000000; // 10^12

        for (int i = 0; i < limit; i++)

        {

            var digit = i;

​

            // Separate digits, i.e 1111 Get's separated to 1,1,1,1

            var separatedDigits = GetSeparatedDigits(i);

​

            // Sum all separated digits, i.e 1+1+1+1

            long sum = separatedDigits.Sum(x => x); 

​

            // Add to global result

            sumOfAllDigits += sum;

        }

​

        Console.WriteLine($"Sum of all digits from 0 to 10^12: {sumOfAllDigits}");

    }

​

    private static List<long> GetSeparatedDigits(long number)

    {

        string numberString = number.ToString();

​

        List<long> digits = new List<long>();

        for (int i = 0; i < numberString.Length; i++)

            digits.Add(long.Parse(numberString[i].ToString()));

​

        return digits;

    }

}