convert raw video .y4m to mpg video with certain GOP

[abeer]

hi,
I want to compress a raw video .y4m to mpg, and I want then to extract the frames from the mpg video, I need the GOP of the compression to be :IBBPBBPBBPBBPBBIBBP....15:2,
I used this command:
ffmpeg -i video.ym4 -vcodec libx264 -sameq -y -r 30 output.avi 2>list.txt
ffmpeg -i output.avi -vcodec libx264 -y -sameq -vf showinfo -y -f image2 image%3d.jpeg -r 30 2>list1.txt

sorry, Im new to ffmpeg, please help me, this is the input to my project, it is the important step to me.

[abeer]

THe output contains only 2 I frames, 100 P and 198 B frames, it is not 15:2 GOP, what to do