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Matrix with 1 & -1

A $n\times n$ matrix has entries $1$ or $-1$. Denote $a_k$ the product of the elements on the line $k$ and $b_l$ the product of the elements on the column $l$. Find all values of $n$ for which we can have $a_1+a_2+...+a_n+b_1+...+b_n=0$.