ffmpeg - Overlay one video on to another

I want to overlay one video on to another.I have two videos with same time duration.

Aim is to make upper video little transparent so that one can see whats happening in background video.

I've tried below command :

String command="ffmpeg -y -i /sdcard/videokit/two.mp4 -i /sdcard/videokit/inone.mp4 -filter_complex [0:v][1:v]overlay=100:100:eof_action=pass[v] -map [v] /sdcard/videokit/output.mp4";

In this command upper video isn't matching(In context of height and width) with video at back.

Confusion : There may be a possibility that background video wont have same height width as upper video cause it comes dynamically in that case how should I manage..!!!

Any help or suggestions would be appreciated.