input: Write a function to generate the Fibonacci series up to a given number. If the given number is negative, print "Incorrect input". If the given number is 0, return 0. If the given number is 1, return 1. For other numbers, return the Fibonacci number at the given position.
  assert fibonacci(0)==0
  assert fibonacci(1)==1
  assert fibonacci(2)==1
  assert fibonacci(8)==21
  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