input: Write a function to calculate the nth Fibonacci number. If n is 0, the function should return 0. If n is 1, the function should return 1. For any other value of n, the function should return the sum of the previous two Fibonacci numbers (i.e., Fibonacci(n - 1) + Fibonacci(n - 2)).
  assert fibonacci(0)==0
  assert fibonacci(1)==1
  assert fibonacci(2)==1
  assert fibonacci(6)==8
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