Yeah, so the thing is that 'e' (or coefficient of restituition) can never be negative. The range of 'e' is only from 0 to 1 depending upon the type of collision that takes place. If it's a perfectly elastic, e = 1 and if it's perfectly inelastic, e=0. All the other values vary differently because the type of collision it undergoes is more or less close to such collisions.Hope I could help.Cheers mate.