Translate position uncertainty of two objects to uncertainty of distance between them

Hello,

I have a plane and in this plane there are two objects. The real location of each object is somewhere in the circle of radius R around the assumed location. R is expressed as one standard deviation.

I know the assumed distance D between both objects (multiples larger than R). Possible real distances vary between the extremas D+-R. Now I would like to express the variation of the distance D as one standard deviation.

Any ideas, links on how to do this in the best way? Thanks!