maths cousework - number grid help

hi, im currently stuck on my maths cousework. i have to find the fomula that works out the difference when a rectangle of any size is placed on an any sized grid. by multiplying opposite corners of the rectangle then subtracting the lowest from the highest number. this get the difference. what i need is a formula that can do this. thanks

I would help if I could understand the question! Could you sketch what you mean and post a picture?