Store previous file # in a variable and compare with the current one.

>
>I want check if a file is missing in a folder.
> For example, below adc99446.txt is missing.
>
>adc99442.txt
>adc99443.txt
>adc99444.txt
>adc99445.txt
>adc99447.txt
>
>I tried subst() and val () to get the number but failed to use for and adir() to scan the files,
>because I found no way use if a - b # 1 inside FOR command.
>
>Could anybody help ?
>
>Moises