Update app.py

app.py

@@ -491,6 +491,7 @@ with gr.Blocks() as demo:
             n_frames_slider = gr.Slider(minimum=1, maximum=9000, value=81, step=1, label="Tracking Frames N")
             track_btn = gr.Button("Tracking")
         video_output = gr.Video(label="Tracking Result", elem_id="my-video")
+        video_input.change(on_video_change, inputs=video_input, outputs=n_frames_slider)
 
         mask_video_input = gr.Video(label="(Optional) Upload Mask Video — white = object to remove")
         use_mask_btn = gr.Button("Use Uploaded Mask (skips clicking & tracking)", elem_id="my-btn")