KMO 2010 Problem 7

There are $2000$ people, and some of them have called each other. Two people can call each other at most $1$ time. For any two groups of three people $A$ and $B$ which $A \cap B = \emptyset$ , there exist one person from each of $A$ and $B$ that haven't called each other. Prove that the number of two people called each other is less than $201000$ .