Black Cells

[from Nikoli website]

1. Place black cells according to the next rules:
2. Each number on the board represents the sum of white cells from that number to black cells or outer frame, horizontally and vertically.
3. The cells which include numbers are always white.
4. Black cells must not touch each other horizontally or vertically.
   
5. All white cells must be orthogonally connected.

Solution: