def f(n): # Check if input is 0 then it will # print incorrect input if n < 0: print("Incorrect input") # Check if n is 0 # then it will return 0 elif n == 0: return 0 # Check if n is 1,2 # it will return 1 elif n == 1 or n == 2: return 1 else: return f(n-1) + f(n-2)

`O(2^n)`