input: Write a function to calculate the Fibonacci numbers up to a given number. If the input number is less than 0, return 0. Otherwise, return the Fibonacci number at the given position.
  def Fibonacci(n): 
  	if n < 0: 
  		return 0
  	if n==0: 
  		return 0
  	if n==1: 
  		return 1
  	else: 
  		return Fibonacci(n-1)+Fibonacci(n-2)
  assert Fibonacci(0)==0
  assert Fibonacci(1)==1
  assert Fibonacci(5)==5
  assert Fibonacci(10)==55
  assert Fibonacci(-1)==0
output: def Fibonacci(n): 
      if n < 0: 
          return 0
      if n == 0: 
          return 0
      if n == 1: 
          return 1
      a, b = 0, 1
      for _ in range(2, n + 1):
          a, b = b, a + b
      return b