How to save frame as 4 digits

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C#

image-recognition

, +

I read a video that has 1000 frames then, I want to save these frames from number 0000 to 1000. I did my code but save the frames from 1,2,3,....1000
how to make all of them have 4 digits, I mean starting with 0001,0002,0003,....1000
Can anyone give me an idea because I don't find anything about my situation?
thanks

What I have tried:

FileName1 = "D:\\Cameras\\VideoCamera1\\output1.mp4";
_capture1 = new VideoCapture(FileName1);
Readframe1 = new Mat();
Readframe1 = _capture1.QueryFrame();
 double framenumber = _capture.GetCaptureProperty(Emgu.CV.CvEnum.CAP_PROP.CV_CAP_PROP_POS_FRAMES);
Readframe1.Save("D:\\cam1\\" + framenumber.ToString() + ".jpg");