- Time limit: 1.00 s
- Memory limit: 512 MB
The first input line has an integer $n$: the size of the grid.
After this, there are $n$ lines that describe the grid. Each line has $n$ characters:
.means an empty square, and
Bshow the characters already placed.
You can assume that every row and column has at most one A and B.
Print one integer: the number of ways modulo $10^9+7$.
- $2 \le n \le 500$