N-queens problem objective function

Question

I have read about the N-queens problem and I don't understand the following objective function:

$O(board)=N^2/2-L$

$N$ ... number of queens $L$ ... number of pairs of queens that attack each other

Why they use $N^2/2$? They wrote that it is the number of pairs of queens but I don't understand that can someone maybe explain it to me maybe with visualization