a little

\(N \times M\) 的网格上放 \(1 \times 3\) 的方块能放 \(\lfloor \dfrac{NM}{3} \rfloor - (n == 2 \ \&\&\ m \% 3 == 0 \ ||\ m == 2 \ \&\&\ n \% 3 == 2)\)