There are two ways to take the second flight of stairs.
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N stairs, if you take the first 1 step, there are n- 1 stairs below.
If you take two steps first, there are n-2 stairs below.
Therefore, the total number of times to take N stairs is the total number of times to take n- 1 stairs plus the total number of times to take n-2 stairs.
That is to say, the number of stairs with 3 steps is equal to 1 step, and the number of stairs with 2 steps is equal to 1+2 = 3.
Four stairs equals the number of two stairs plus three stairs, that is, 2+3 = 5.
5 Stairs 3+5 = 8 kinds
6 Stairs 5+8 = 13 kinds.
7 Stairs 8+ 13 = 2 1 species.
8 Stairs 13+2 1=34 9 Stairs 2 1+34=55 Stairs 10 34+55=89 Stairs and so on, that is, the number of the next item is the equal sign of the previous item plus the number of the previous item.