input: Write a python function to get the nth Fibonacci number.
  assert get_fibonacci_number(0) == 0
  assert get_fibonacci_number(1) == 1
  assert get_fibonacci_number(5) == 5
  assert get_fibonacci_number(10) == 55
output: def get_fibonacci_number(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