P2181 Diagonals

For an $n$-vertex convex polygon in which no three diagonals intersect at a single point, find the number of intersection points among its diagonals. For example, a $6$-gon: 

The input contains a single line with an integer $n$, the number of sides.

Output a single integer on one line: the answer.

Constraints - For $50 \%$ of the testdata, it is guaranteed that $3 \leq n \leq 100$. - For $100 \%$ of the testdata, it is guaranteed that $3 \leq n \leq 10^5$. Translated by ChatGPT 5