```
m = picture.size
n = picture[0].size
row = Array.new(m,0)
col = Array.new(n,0)
(0...m).each do |i|
(0...n).each do |j|
if picture[i][j] == 'B'
row[i] += 1
col[j] += 1
end
end
end
res = 0
(0...row.size).each do |i|
(0...col.size).each do |j|
if row[i] == 1 && col[j] == 1 && picture[i][j] == 'B'
res += 1
end
end
end
return res
```