[FFmpeg-user] timelapse framerates

[Michael Koch]

>> ffmpeg -framerate 25 -start_number 1 -i IMG_%4d.jpg
>> -codec:v mpeg4 output.mp4
> Please understand that you are expected to always
> post your command line including the complete,
> uncut console output on this mailing list.

The above command line does exactly what it's supposed to do.
Let's assume I have 250 input pictures. With framerate 25, the video 
length is 10 seconds.

My questions (with other words) are:
How must I modify the command line, if I want a video with framerate 25, 
but 20 seconds long? That means each picture must be used twice: 
1,1,2,2,3,3,4,4, ...
How must I modify the command line, if I want a video with framerate 25, 
but only 5 seconds long? That means all pictures with even numbers must 
be skipped: 1,3,5,7,9, ...

Thanks,
Michael