using System;

​

public class Hex

{

    private static int CharToInt(char c)

    {

        if ('0' <= c && c <= '9')

        {

            return c - '0';

        }

        if ('a' <= c && c <= 'f')

        {

            return 10 + c - 'a';

        }

        throw new ArgumentOutOfRangeException(nameof(c));

    }

​

    static string IntToCharMap = "0123456789abcdef";

​

    private static char IntToChar(int i)

    {

        return IntToCharMap[i];

    }

​

    public static string Add(string str1, string str2)

    {

        var result = new char[Math.Max(str1.Length, str2.Length) + 1];

        int overflow = 0;

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

        {

            int i1 = str1.Length - i - 1;

            int i2 = str2.Length - i - 1;

            int n1 = (0 <= i1) ? CharToInt(str1[i1]) : 0;

            int n2 = (0 <= i2) ? CharToInt(str2[i2]) : 0;

            int sum = n1 + n2 + overflow;

            overflow = sum / 16;

            result[result.Length - i - 1] = IntToChar(sum % 16);

        }

        if (result[0] == '0')

        {

            return new String(result, 1, result.Length - 1);

        }

        return new String(result);

    }

}

public class Program

{

    public static void Main()

    {

        Console.WriteLine(Hex.Add("fed", "13"));

    }

}