How to remove multiple frames from gif after import?

So I imported a video and immediately told it to remove every second frame during import.  I edited my animation -- cropped, colored, resized, etc., and now realize that I still need to reduce half the frames (from 500 to 250 or so).  When you import the video originally, it asks if you want to choose to remove frames, but how do you do it once you have the frames already in the timeline?  It doesn't make sense to CTRL-Click 250 frames.  Is there no easy way?

Thanks!!