input: Write a function to calculate the nth Fibonacci number. The function should return the nth number in the Fibonacci sequence, where the first two numbers are 0 and 1, and each subsequent number is the sum of the previous two. If the input is 0, the function should return 0. If the input is 1, the function should return 1. For any other input n, the function should return the sum of the (n-1)th and (n-2)th Fibonacci numbers.
  assert fibonacci(0) == 0
  assert fibonacci(1) == 1
  assert fibonacci(6) == 8
  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