Calculate distance between keypoints in OpenPose?

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I am trying to get the distance between keypoints obtained from Openpose in real-world measurements. I have an android application that takes the images and sends it to my backend, where I calculate the keypoints using body_25 model. The issue is I want to convert them to real world measurements for an E-Dressing platform. I can get the focal length and probably the distance of the object from the camera using my android application. I just wanna know how I can calculate the distance between keypoints. The keyponits being, "LeftShoulder", "LeftWrist".