input: Write a function to print the Fibonacci series up to a given number `n`. If `n` is negative, print "Incorrect input". If `n` is 0, return 0. If `n` is 1, return 1. Otherwise, return the Fibonacci number at position `n`.
  assert fibonacci(0)==0
  assert fibonacci(1)==1
  assert fibonacci(2)==1
  assert fibonacci(5)==5
  assert fibonacci(10)==55
output: def fibonacci(n):
      if n < 0:
          return "Incorrect input"
      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