input: Write a function to calculate the Fibonacci numbers up to a given number. The function should return the nth Fibonacci number. If the given number is 0, return 0. If the given number is 1, return 1. For any other number, return the sum of the two preceding Fibonacci numbers.
  assert Fibonacci(0)==0
  assert Fibonacci(1)==1
  assert Fibonacci(5)==5
  assert Fibonacci(10)==55
  assert Fibonacci(15)==610
output: def Fibonacci(n):
      if 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