input: 
  from typing import List
  
  def max_revenue(prices: List[int]) -> int:
      """ Calculate the maximum revenue achievable by buying and selling stocks.
      The input 'prices' is a list of stock prices where the index represents the day.
      The function finds the maximum possible profit by considering all possible pairs of buy and sell days.
      >>> max_revenue([10, 7, 5, 8, 11, 9])
      6
      >>> max_revenue([100, 180, 260, 310, 40, 535, 695])
      655
      """
      diff = [prices[i + 1] - prices[i] for i in range(len(prices) - 1)] 
      max_diff = 0
      for i in range(len(diff)): 
          for j in range(i + 1, len(diff) + 1): 
              current_diff = sum(diff[i:j]) 
              if current_diff > max_diff: 
                  max_diff = current_diff 
      return max_diff
  
output:     diff = [prices[i + 1] - prices[i] for i in range(len(prices) - 1)] 
      max_diff = 0
      current_sum = 0
      for d in diff:
          current_sum = max(0, current_sum + d)
          max_diff = max(max_diff, current_sum)
      return max_diff