Pairs of People No.1

[rsoldo]

In a set of $n$ people, there are not 3 people such that every two of them know each other. Prove that the number of pairs of people who know each other is not greater than $\left\lfloor \frac{n^2}{4}\right \rfloor$.

[Edward_Tur]

https://en.wikipedia.org/wiki/Tur%C3%A1n%27s_theorem