Chessboard 5x5 with twentyfour"+1" and one "-1"

Question

A number $+1$ has been entered in 24 fields of the $5 \times 5$ array, and the number $-1$ in one.
In one move, you can multiply by $ -1 $ all numbers in the subarray $ k \times k $ where $ k> 1 $.
Which field at the beginning contained the number $ -1 $, since after a finite number of moves you can get to the state with twentyfive $ + 1 $


I have already proved that that $-1$ cannot be written in fields with $a$ since every subarray contains even number of fields with the letter $a$, therefore the product of the numbers in fields with letter $ a $ would be negative, which leads to a contradiction. \begin{matrix} b & b & a & b & b \\ b & b & a & b & b \\ a & a & 0 & a & a \\ b & b & a & b & b \\ b & b & a & b & b \end{matrix} However I'm not able to to disprove the possibility of $-1$ in fields with letter $b$.
Any help would be greatly appreciated.