题意

有一个有 $n=2k$ 个点的无向图 $G$，初始时有一些边，每次可以选择一个四元环 $a,b,c,d$，若 $\left(a,c\right)$ 这条边不存在，则加入这条边。问：初始时最少有多少条边，使得进行有限次操作后，$G$ 可以变成完全图。

There are 2022 users on a social network called Mathbook, and some of them are Mathbook-friends. (On Mathbook, friendship is always mutual and permanent.) Starting now, Mathbook will only allow a new friendship to be formed between two users if they have at least two friends in common. What is the minimum number of friendships that must already exist so that every user could eventually become friends with every other user?