integers into strings

[isaac_2004@yahoo.com]

hi im new to ada and i want to take a number inputted from a user and
seperate the digits then arrange it in ascending and descending order
and subtract and ascending number from the descending number

for example

user inputs 7319

program outputs   9731 - 1379 = 8352

8532 - 2358 = 6174
 and so on


also loop it until number acquired after subtraction is the same as
starting number

like this
6174
7641 - 1467 = 6174  

what procedure would i use to do this