# You are given a positive integer N which represents the number of steps in a staircase. You can either climb 1 or 2 steps at a time. 
# Write a function that returns the number of unique ways to climb the stairs.

def staircase(n):
  # Fill this in.
  
print staircase(4)
# 5
print staircase(5)
# 8

# Can you find a solution in O(n) time?