imporve the the quality of merged video ffmpeg flutter

Question

I used this command to merge two video clips, but the quality of output is too low. I have not used ffmpeg before I read about parameter -crf 27 and -preset veryfast to improve the quality but dont know how to use them with this command. Any help would be appreciated.

'-y -i ${_storedVideoOne.path} -i
 ${_storedVideoTwo.path} -r 24000/1001 -filter_complex
 \'[0:v:0][0:a:0][1:v:0][1:a:0]concat=n=2:v=1:a=1[out]\' -map \'[out]\'
 $outputPath';