One: there is only one level at a time, and there is one.

Two: one crossing the second level, the other eight crossing the first level, C9 1=9.

Three: two× 2 and one× 6, C82=28.

Four: 2× 3 1× 4, C73=35.

V: 2× 4 1× 2, C64= 15.

Six: two times five, one kind.

*** 1+9+28+35+ 15+ 1=89