input: def fibonacci(n):
      if n == 1 or n == 2:
          return 1
      else:
          return fibonacci(n - 1) + fibonacci(n - 2)
  
  print(fibonacci(10))
output:     if n == 1 or n == 2:
          return 1
      else:
          a, b = 1, 1
          for _ in range(3, n + 1):
              a, b = b, a + b
          return b