KöMaL 2019 B5041

An $n \times n$ table of real numbers in each field is called a zero square if the sum of the numbers in every $2 \times 2$ square part of it (therefore in the whole table, too) is zero. (The diagram shows a $3\times3$ example.) 2 -3 4 -4 5 -6 1 -2 3 What is the largest possible $n$ for which there exists an $n \times n$ zero square such that the entries are not all zeros?