## Defensive Bishop

#### Problem

In a game of chess, the Bishop can take any piece that lies on a diagonal.

By placing five Bishop pieces on a 4 by 4 grid every square is protected in such a way that no Bishop threatens any other Bishop.

However, it is possible to achieve the same with just four bishops.

What is the least number of Bishop pieces required to protect every square on a standard 8 by 8 chessboard?

Problem ID: 144 (Jan 2004) Difficulty: 1 Star