input: Write a function to implement a basic algorithm in Python to calculate the nth Fibonacci number. The function should return the Fibonacci number for the given input. If the input is 0, return 0. If the 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(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