@echo off&setlocal enabledelayedexpansion
for /f "delims=" %%i in (a.txt) do (
set /a m+=1
set _!m!=%%i
)
set nn=0
set mm=-1
:sta
set /a nn+=2
set /a mm+=2
if defined _%nn% echo.!_%nn%! 
if defined _%mm% echo.!_%mm%!
if !nn! lss !m! goto sta 
pause  
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