-r[:stream_specifier] fps (input/output,per-stream)
           Set frame rate (Hz value, fraction or abbreviation).

           As an input option, ignore any timestamps stored in the file and
           instead generate timestamps assuming constant frame rate fps.  This
           is not the same as the -framerate option used for some input
           formats like image2 or v4l2 (it used to be the same in older
           versions of FFmpeg).  If in doubt use -framerate instead of the
           input option -r.