P3524 [POI 2011] IMP-Party

本题数据极大，评测时可能需要较长时间等待。

```plain Byteasar intends to throw up a party. ``` Naturally, he would like it to be a success. Furthermore, Byteasar is quite certain that to make it so it suffices if all invited guests know each other. He is currently trying to come up with a list of his friends he would like to invite. Byteasar has $n$ friends, where $n$ is divisible by 3. ```plain Fortunately, most of Byteasar's friends know one another. ``` Furthermore, Byteasar recalls that he once attended a party where there were $\frac{2}{3}n$ of his friends, and where everyone knew everyone else. Unfortunately, Byteasar does not quite remember anything else from that party. In particular, he has no idea which of his friends attended it. Byteasar does not feel obliged to throw a huge party, but he would like to invite at least \frac{n}{3} of his friends. He has no idea how to choose them, so he asks you for help.

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