a b a+b a-b a+b+1 a-b-1 a-b+4
5----4------9..............1................9+1=10...........1-1=0.............1+4=5
3......2.........5..............1................5+1=6.............1-1=0............1+4=5
4.......0.........4..............4...............4+1=5..............4-1=3...........4+4=8
7........2........9...............5...............9+1=10...........5-1=4...........5+4=9
