input: Write a function to calculate the nth number in the Fibonacci sequence. If the input n is less than 0, print "Incorrect input" and return None. Otherwise, return the nth Fibonacci number. The Fibonacci sequence is defined as follows: F(0) = 0, F(1) = 1, and for n > 1, F(n) = F(n-1) + F(n-2). 
  assert fibonacci(0)==0
  assert fibonacci(1)==1
  assert fibonacci(2)==1
  assert fibonacci(3)==2
  assert fibonacci(4)==3
  assert fibonacci(9)==34
output: def fibonacci(n):
      if n < 0:
          print("Incorrect input")
          return None
      elif n == 0:
          return 0
      elif n == 1:
          return 1
      else:
          a, b = 0, 1
          for _ in range(2, n + 1):
              a, b = b, a + b
          return b