Illuminated boxes

Arun placed a laser on a sheet making **45 Degree** with both the edges at the $top$ $left$ $corner$. The sheet is a rectangle with dimensions of length $n$ and width $m$. Then he divided the sheet into $n*m$ boxes of size $1*1$. He then put mirrors along all edges of the sheet facing inward so that the laser reflects accordingly. He considers the box illuminated if the laser passes through it.