input: 
  def fibonacci(n):
      """Compute the nth Fibonacci number.
      The Fibonacci sequence is defined such that each number is the sum of the two preceding ones.
      
      >>> fibonacci(1)
      1
      >>> fibonacci(2)
      1
      >>> fibonacci(10)
      55
      """
      if n == 1 or n == 2:
          return 1
      else:
          return fibonacci(n - 1) + fibonacci(n - 2)
  
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