MathNet 2019-part-02 — 184 problems (EN–VI) Question 4

Each cell of an $n \times n$ table is filled with one of the two signs: «+» and «-». For each $k$ from $1$ to $n$ the amount of pluses in the first $k$ rows is greater than the amount of minuses in the first $k$ columns.
Find the maximal possible number of minuses in the table.