How many matrices in which product of numbers in each row and column is 1? 
We have a matrix $S$. Every $S_{ij}$ is equal to $1$ or $-1$. How many matrices we can have so that product of all entries in every row and column is $1$?

This means number of $-1$ must be even in all rows and columns. And size of $S$ is $m\times n$.
My idea was that we can divide a matrix of $(m-1)\times (n-1)$, then count all the possible ways this matrix can have $1$ and $-1$ which is $2^{(m-1)(n-1)}$.
Then add other part of matrix and set those values in a way that number of $-1$ become even. But this way doesn't work.
Sorry if I didn't explain my way very clearly, it's because English is not my native language.
 A: Consider an example :
$$
\begin{array}{|c|c|c|c|c|c|c|c|}
\hline
1 & -1 & -1 & -1 & 1 & 1 & -1 & \color{blue}{1}\\ \hline
1 & -1 & 1 & -1 & 1 & -1 & 1 & \color{blue}{-1}\\ \hline
-1 & 1 & 1 & 1 & 1 & -1 & -1 & \color{blue}{-1}\\ \hline
-1 & 1 & -1 & -1 & 1 & -1 & 1 & \color{blue}{1}\\ \hline
1 & 1 & -1 & -1 & -1 & 1 & 1 & \color{blue}{-1}\\ \hline
\color{blue}{1} & \color{blue}{1} & \color{blue}{-1} & \color{blue}{1} & \color{blue}{-1} & \color{blue}{-1} & \color{blue}{1} & \color{red}{-1}\\ \hline
\end{array}
$$
The table is a valid $m\times n$ matrix. Black entries denote $(m-1)\times (n-1)$ matrix.
Key observation :

*

*Last entry (in any row/column) = Product of all other entries (in respective row/column)

So all the last entries (colored) are uniquely determined, once entries of $(m-1)\times (n-1)$ matrix are arbitrarily chosen. Hence answer is $$2^{(m-1)(n-1)}$$
Note the peculiarity about corner entry.
Product of last column blue entries = Product of all black entries = Product of last row blue entries = Red entry
