input: Write a function to calculate the nth number in the Fibonacci sequence. The function should return the nth Fibonacci number. If the input n is less than 0, the function should print "Incorrect input" and return None. If the input n is 0, the function should return 0. If the input n is 1, the function should return 1. For any other positive integer n, the function should return the nth Fibonacci number by calculating iteratively.
  
  assert fibonacci(9)==34
  assert fibonacci(0)==0
  assert fibonacci(1)==1
  assert fibonacci(-1)==None
  assert fibonacci(5)==5
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