input: Write a function to calculate the nth Fibonacci number. If the number is 0, return 0. If the number is 1, return 1. For any other number, return the sum of the two preceding numbers in the Fibonacci sequence.
  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