FFMPEG – How to reduce frame rate without slow motion effect?

I have an MP4 video that’s 21 minutes long, that was shot on a GoPro Hero 9 Black at 2.7k and 240 frames per second using HEVC.

I would like to use FFMPEG to reduce the frame rate, with the output being a video that is also 21 minutes long and 2.7k … but with a frame rate of 120 frames per second. But when I use the command “ffmpeg -i input.mp4 -filter:v fps=120 output.mp4 I get a video that is 42 minutes long.

I would like to tell FFMPEG to simply delete every other frame of the input video, while keeping the total length of the video the same. What command would I use to do that?

Also, how would I modify this command if I wanted the output to be encoded in AVC?

Thank you.