input: Write a function to calculate the nth Fibonacci number. If the input n is 0, return 0. If the input n is 1, return 1. For any other value of n, 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