How to calculate number of tiles so we get 81 tiles on right and left diagonals in 2D matrix?

Question

https://code.sololearn.com/cGgBkTnq935O/?ref=app

Answer 1

The constants must be initialized and you cannot repeat 2 constants with the same name

Error in:
const int M;
const int M;

Try this:
const int M=3;
const int N=3;

Answer 2

Remember that a constant cannot change its value once started.

Lo mejor seria que uses:
int M=5

Sin la palabra const

Answer 3

I try to solve it with trying and get correct result. I would like to know how to solve it with code.