I want to add a crossfade between two videos with ffmpeg [duplicate]

Question

I've tried many different things for this to work. the final thing i tried was

ffmpeg -i BabySeal.mp4 -i Pexels2.mp4 -filter_complex " [0:a]afade=t=out:st=9:d=2[a0]; [1:a]afade=t=in:st=0:d=2[a1]; aevalsrc=0:d=9[s1]; [s1][a1]concat=n=2:v=0:a=1[ac1]; [a0][ac1]amix[a]" -map [a] -c:a libfdk_aac -ac 2 -b:a 128k output_audio "transition3.mp4"

I don't need there to be audio since the files dont have audio, but im new to ffmpeg and have no idea what anything means.

I've also tried

ffmpeg \ -loop 1 -t 5 -i 1.png \ -loop 1 -t 5 -i 2.png \ -loop 1 -t 5 -i 3.png \ -loop 1 -t 5 -i 4.png \ -loop 1 -t 5 -i 5.png \ -filter_complex \ "[1]format=yuva444p,fade=d=1:t=in:alpha=1,setpts=PTS-STARTPTS+4/TB[f0]; \ [2]format=yuva444p,fade=d=1:t=in:alpha=1,setpts=PTS-STARTPTS+8/TB[f1]; \ [3]format=yuva444p,fade=d=1:t=in:alpha=1,setpts=PTS-STARTPTS+12/TB[f2]; \ [4]format=yuva444p,fade=d=1:t=in:alpha=1,setpts=PTS-STARTPTS+16/TB[f3]; \ [0][f0]overlay[bg1];[bg1][f1]overlay[bg2];[bg2][f2]overlay[bg3]; \ [bg3][f3]overlay,format=yuv420p[v]" -map "[v]" -movflags +faststart out.mp4

I got this from here Create video with 5 images with fadeIn/out effect in ffmpeg

is there something wrong with the code that i put in, or maybe am i doing something else wrong. The file names are correct, so could it be that i'm just missing a step?