To reach n, i should first reach n-2 or n-1; and to reach n-2, i should reach n-3 or n-2.
Considering this, i will get:

F(n) = F(n-1)+F(n-2)
F(n-1) = F(n-2)+F(n-3)
...
F(2)=2
F(1)=1

This means to reach the n-th step, the number of ways is a Fibonacci number F(n).