@Echo off
set ffmpeg_path="C:\Program Files\ffmpeg\bin\ffmpeg.exe"
for /r %%f in (*.mp4) do (
set input_file=%%f
set start_time=-50
set end_time=-35
%ffmpeg_path% -i "%input_file%" -ss %start_time% -to %end_time% -c:v copy -c:a copy "%input_file%OK.mp4"
)