If Alice has x, y, Bob has x, z and Carol has y, z can they determine if x+y+z=N? They can if (say) Alice broadcasts x to Bob and Carol; can they do better? Chandra, Furst, and Lipton studied this problem and showed sublinear upper bounds.

They also had matching (up to an additive constant) lower bounds. We give an exposition of their result with some attention to what happens for particular values of N.