input: Write a function to calculate the nth Fibonacci number. The function should return the Fibonacci number for the given input n. If the input is 0, the function should return 0, and if the input is 1, the function should return 1. For any other input, the function should 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(3)==2
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