Given an m $ imes$ n grid picture consisting of characters 'W' (white) and 'B' (black), return the number of black lonely pixels.

A black pixel is considered lonely if and only if it is the only 'B' in its entire row and the only 'B' in its entire column.

Constraints:

• m == picture.length

• n == picture[i].length

• $1 \leq$ m, n $\leq 500$

• picture[i][j] is 'W' or 'B'.

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