Adding integers

Ride the lift in Bela's Building of Fun. Start at the Food Court, Floor +1, and press +2. You go up two floors and reach Floor +3, the Book Store. We write this as an addition:

• Starting Floor + Movement = Target Floor, so (+1) + (+2) = +3.

Movements can go down too. Start on Floor +2 and press -3: two floors down brings you to Floor 0, and one more brings you to Floor -1. So (+2) + (-3) = -1. In the same way, starting at Floor +2 and pressing -7 takes you to Floor -5: (+2) + (-7) = -5. And 0 + (-2) = -2: from the ground floor, two floors down.

Combining button presses is also addition. Gurmit was in the Toy Store and wanted to go down two floors, but by mistake he pressed '+' twice. He quickly pressed '-' three times. Up 2 and then down 3 is one floor down altogether: (+2) + (-3) = -1. So Gurmit ends up one floor below the Toy Store.

You can add a whole string of presses the same way, one press at a time. (+4) + (-3) + (-2): up 4, down 3 (now 1 up), down 2 more, so the total movement is -1.