using System;

​

public class Program

{

    public static void Main()

    {

        byte[] f = new byte[] { 1, 1, 1 };

        byte[] s = new byte[] { 255, 1, 1 };

        foreach (byte r in addArrays(f, s))

        {

            Console.Write(r);

        }

    }

​

    public static byte[] addArrays(byte[] f, byte[] s)

    {

        int n = f.Length;

        byte[] result = new byte[n];

        int carryOver = 0;

        for (int i = n - 1; i >= 0; i--)

        {

            int sum = f[i] + s[i] + carryOver;

            if (sum > 255)

            {

                result[i] = (byte)(sum - 256);

                carryOver = 1;

            } 

            else

            {

                result[i] = (byte)sum;

                carryOver = 0;

            }

        }

        if (carryOver == 0)

            return result;

        else

        {

            // Oh oh, last addition has caused carry over.

            byte[] extendedResult = new byte[n+1];

            extendedResult[0] = 1;

            Array.Copy(result, 0, extendedResult, 1, n);

            return extendedResult;

        }

    }

}