@echo off&setlocal enabledelayedexpansion
(for /f "delims=" %%a in (a.txt) do (
  set /a n+=1
  set "str=%%a"
  if !n! lss 10 (echo,!str:~,-1!!n!
  ) else (
    if !n! lss 100 (echo,!str:~,-2!!n!
    ) else if !n! lss 1000 echo,!str:~,-3!!n!
  )
))>b.txt
pause
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