# Problem B

Piece It Together

Tom has developed a special kind of puzzle: it involves a whole bunch of identical puzzle pieces. The pieces have the shape of three adjoint squares in an L-shape. The corner square is black, the two adjacent squares are white.

The puzzler is given a pattern of black and white squares in a rectangular grid. The challenge is to create that pattern using these pieces. The pieces can be rotated, but must not overlap.

Tom has already designed a few nice patterns, but he needs to find out if they can be constructed with the pieces at all. Rather than trying to test this for each pattern by hand, he wants to write a computer program to determine this for him. Can you help him?

## Input

The first line contains two integers $n$ and $m$ ($1
\leq n, m \leq 500$): the height and width of the grid
containing the pattern, respectively. The next $n$ lines contain $m$ characters each, denoting the
grid. Each character is ‘`B`’,
‘`W`’, or ‘`.`’, indicating a black, white or empty square
respectively. The grid contains at least one black or white
square.

## Output

Output “`YES`” or “`NO`”, indicating whether or not it is possible
to construct the pattern with the puzzle pieces. You may assume
that there is an infinite supply of pieces.

Sample Input 1 | Sample Output 1 |
---|---|

3 4 BWW. WWBW ..WB |
YES |

Sample Input 2 | Sample Output 2 |
---|---|

3 3 W.. BW. WBW |
NO |