Lazy caterer's sequence

Lazy caterer's sequence is a way of counting the number of pieces that you would get if you cut a pizza into different shapes. For example, if you cut a pizza into 3 slices, you could get 3 pieces. If you cut the same pizza into 4 slices, you will get 7 pieces. The next time you cut it into 5 slices, you'll get 12 pieces. And going on and on, you'll find that you will always get the same number of pieces as the number of slices, plus the number before it added together. So, it looks like this:
1 slice = 1 piece
2 slices = 1 + 2 = 3 pieces
3 slices = 3 + 3 = 6 pieces
4 slices = 6 + 4 = 10 pieces
5 slices = 10 + 5 = 15 pieces
6 slices = 15 + 6 = 21 pieces
... and so on.