奇偶性前提是定义域关于原点对称；结合单调性解不等式。
How many people do we need to have in a room to make it a favorable bet (probability of success greater than 1/2) tha…
一个教室里随便点 23 个人，两两生日撞上的概率刚过半。直觉猜 183，真答案是 23。
