As a stock exchange broker focusing on a single transaction, you aim to maximize your profit by buying a stock at a lower price and selling it later at a higher price within a given period.

Stock Profit
Your function needs to analyze the price fluctuations of the stock over time. It should identify the most profitable opportunity by calculating the maximum potential profit achievable within these fluctuations. This means finding the highest price to sell the stock after buying it at the lowest possible price.
However, if there's no possibility to make a profit—such as when the stock price consistently decreases or remains the same—the function should return zero, indicating that there's no viable opportunity for a profitable transaction.
Input: Stock prices as list of integers (int).
Output: Maximum possible profit as an integer (int).
Examples:
assert stock_profit([2, 3, 4, 5]) == 3 assert stock_profit([3, 1, 3, 4, 5, 1]) == 4 assert stock_profit([4, 3, 2, 1]) == 0 assert stock_profit([6, 2, 1, 2, 3, 2, 3, 4, 5, 4]) == 4
Preconditions:
- len(stock) > 1;
- 0 <= price <= 1000.