Adding a Large number digit by digit

I posted a previous thread but it is pretty much dead, so here is a new one.

I'm trying to add to very large numbers digit by digit that the user inputs.
I have it working as long as the numbers are the same length digit wise and the last number does not have a carry.

I'm not sure how to shift the numbers and keep the program working the way its suppose to.

Code:

#include "stdafx.h"
#include<stdio.h>
#include<string.h>
#include<stdlib.h>

int main(int argc, char* argv[])
{
unsigned char number1 [256] = {'0'};
unsigned char number2 [256] = {'0'};
unsigned char result [256] = {'0'};
int i, y=0, sum=0, carry=0;

printf ("Please enter a number upto 255 digits long:\n");
scanf ("%s", number1);
printf ("Your first number is %s.\n",number1);

printf ("Please enter another number upto 255 digits long:\n");
scanf ("%s", number2);
printf ("Your second number is %s.",number2);


memset (result, 0, sizeof(result));


	for(y=255; y>=0; y--) {
		
BACK:		
		if( number1[y] == 0) {
			y--;
			goto BACK;
		}
		else if( number2[y] == 0) {
			y--;
			goto BACK;
		}
		else{
		sum += carry;
		sum += number1[y]-'0'+number2[y]-'0';
		carry = sum/10;
		result[y] = sum%10+'0';
		sum = 0;}
}
  printf("\nThe sum is %s.", result);


  return 0;
}

Thank you for your help.

There's still life in your other thread, and starting a new one just looks like bumping.