input: Write a function to print the Fibonacci series up to a given number n. The function should return the Fibonacci number at position n. If the input number is less than 0, it should print "Incorrect input" and return None.
  def fibonacci(n):
      a = 0
      b = 1
      if n < 0:
          print("Incorrect input")
          return None
      elif n == 0:
          return a
      elif n == 1:
          return b
      else:
          for i in range(2, n + 1):
              c = a + b
              a = b
              b = c
          return b
  
  assert fibonacci(0) == 0
  assert fibonacci(1) == 1
  assert fibonacci(5) == 5
  assert fibonacci(10) == 55
  assert fibonacci(-1) is None
output: def fibonacci(n):
      a = 0
      b = 1
      if n < 0:
          print("Incorrect input")
          return None
      elif n == 0:
          return a
      elif n == 1:
          return b
      else:
          for i in range(2, n + 1):
              c = a + b
              a = b
              b = c
          return b