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\u003c/script\u003e\n\u003ch1\u003e题目描述\u003c/h1\u003e\n\u003cp\u003e你和你的朋友喜提亲子餐。\u003c/p\u003e\n\u003cp\u003e在路上，你们突然间意识到了一个十分棘手的问题：谁是爸爸？之所以只需要考虑爸爸的人选，是因为你的朋友花果在第一时间跳了出来选择当妈妈。剩下你和你的朋友面面相觑。你们最终决定通过一个游戏来一决高下：\u003c/p\u003e\n\u003cp\u003e花果给你们二人每人发了1~n的n个数字，你们需要玩一共n轮游戏。每轮从自己\u003cb\u003e还未出过\u003c/b\u003e的数字当中选取一个，两人同时公布。其中数字较大的人得到与其出的数字相等的分数，数字较小的人在这一轮不获得分数，如果二人数字相等，则二人均不获得分数。具体而言，如果在一轮当中我出了10，我的朋友出了7，那么这一轮我就能获得10分，我的朋友在这一轮就无法获得分数。注意，一个数字在出过了之后就视为作废，在接下来的游戏当中出它的人就不能够再继续使用。（双方并不干扰，也就是说，我出了10不意味着你无法再出10。）\u003c/p\u003e\n\u003cp\u003e玩到一半，你顿悟了。无论谁输谁赢，你们之间都是逃不掉的血浓于水的父子情！那又有什么分别呢？于是，你不再在乎游戏的输赢，转而关心一个更为复杂的问题：假如每一轮你都与你的儿子随机选择数字，那么在结束n轮游戏之后，你所获得的分数之和期望为多少呢？\u003c/p\u003e"}},{"title":"","value":{"format":"HTML","content":"\u003ch2\u003e输入格式\u003c/h2\u003e\n\u003cp\u003e输入一行，一个整数n，表示游戏的轮数。其中1\u003c\u003dn\u003c\u003d1e6。\u003c/p\u003e"}},{"title":"","value":{"format":"HTML","content":"\u003ch2\u003e输出格式\u003c/h2\u003e\n\u003cp\u003e输出一个实数，表示n轮游戏后你所获得分数之和的期望值。 \u003c/p\u003e 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