Because you can climb one level or jump two levels at a time. You can jump up to three levels.

So the ladder mode of n = ladder mode of n-65438+ladder mode of 0+n-2+ladder mode of n-3.

So climb 1 level 1.

Climb the second floor 2

Climb the third floor and the fourth floor

Climb the 4th and 7th floors.

Climb the fifth floor 13

Climb level 6 24

Climb level 7 44

Climb level 8 8 1