Two grandkids Dorothy and Benjamin are sharing a telescope made by their grandpa, who used an old lens of spectacles as the objective. This objective lens has power of p diopter. Dorothy definitively watches a doghouse at distance d meters. Benjamin basically watches a birdhouse at distance b meters. After watching the doghouse, how much shift of the eyepiece in mm is necessary to watch the birdhouse? Please give a general solution, the test suite may be more extensive later.