Please reason step by step, and put your final answer within \boxed{}.
Fill in the nine algebraic expressions into the squares of the grid, so that the sum of the three algebraic expressions in each row, each column, and both diagonals of the grid are equal. Given some of the algebraic expressions in the grid as shown in the table below, find $M-N$ ($M$ and $N$ are algebraic expressions containing $x$).

|  $M$  |        | $x^{2}-x-1$ |
|-------|--------|-------------|
|       |        | $x$         |
| $x^{2}-x$ | $x-1$ | $N$         |
user