How can i save only one image using the imwrite command without press any key?

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I'm using OpenCV for facial recognize and i want to save automatically only one image when the person is detected in front of webcam withou press any key.

In this case i want to save only one image and the command imwrite is saving multiple images. useless screenshot

here is my code. it's detecting the face in front of the camera,when is a unknown person in front of the camera it returns -1 and i want to save a screenshot from the camera using the imwrite command but it's saving lots of pictures.