input: Write a python function to calculate the n-th Fibonacci number.
  assert calculate_fibonacci(0) == 0
  assert calculate_fibonacci(1) == 1
  assert calculate_fibonacci(10) == 55
  assert calculate_fibonacci(15) == 610
output: def calculate_fibonacci(n):
      if n == 0:
          return 0
      elif n == 1:
          return 1
      
      a, b = 0, 1
      for _ in range(2, n + 1):
          a, b = b, a + b
      return b