input: Write a function to find the nth Fibonacci number. If the input is less than 0, print an incorrect input message. Otherwise, return the nth Fibonacci number.
  
  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:
          print("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