Why is ffmpeg not working?

Question

fmpeg

ffmpeg  -i 2.aac -i 3.aac  -filter_complex "[0]asetpts=0;[1]asetpts=8000; concat=n=2:a=0:a=1 [aout]" -map "[aout]" out.aac

I'm trying to connect 2 aac files, the second one for 8 seconds

gives:
Cannot find a matching stream for unlabeled input pad 0 on filter Parsed_concat_2

Answer 1

ffmpeg -i 2.aac -i 3.aac -i 4.aac -filter_complex "[0]adelay=0[a0];[1]adelay=2000[a1];[2]adelay=1000[a2];[ a0][a1][a2]amix=inputs=3" out.aac