Finding the circumference of a shape found by findContours

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I am trying to find a red bottle cap and want to use the findContours function together with moments. I know that i get the area of the contour when i access m00 of the moment. I would like to know if there is a way to calculate the circumference of a shape, if there is already a function that handles such a thing or if there is another attribute which i simply have to read.

I looked here but was not able to find something fitting.