input: Write a function to calculate the nth Fibonacci number. The function should return the Fibonacci number for the given input. If the given input is 0, return 0. If the given input is 1, return 1. For any other input, return the sum of the two preceding Fibonacci numbers. 
  
  assert fibonacci(6) == 8
  assert fibonacci(0) == 0
  assert fibonacci(1) == 1
  assert fibonacci(10) == 55
  assert fibonacci(2) == 1
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