Fly Time

A fly $F$ tired of flying wants to sit on a chess board of dimensions $n×m$ . Each cell of the chess board is a square of size $1×1$ $cm^2$ . The flies of Chef land are weird they can only fly horizontally or vertically. Given that this fly can fly exactly $C$ centimeters in vertical or horizontal direction and after that have to sit on the board to rest for a bit and then continue her journey fur