Let's play a dice game. You roll a fair 6-sided die and receive dollars equal to the number rolled. You can choose to reroll once if you don't like the first outcome, but you must keep the second result. What is your optimal strategy and the expected payoff?

我们来玩个掷骰子的游戏。你掷一个标准的六面骰子，结果是几你就能得几块钱。如果你对第一次的结果不满意，可以选择重掷一次，但必须接受第二次的结果。你的策略是什么？这个游戏的期望收益是多少？