Advices about the approach ?

I have problem understanding this task, that i have for examen:

You have to write a program in C++ that computes the minimal and the maximal value of function f(x,y) obtained on an integer point in a given rectangle [a, b] x [c, d]. Your program should prompt the user to input numerical values of a, b, c and d, as floating point numbers, which are expected to be in a range from -100 thru 100. In case when minimal or maximal values do not exists, your program should output appropriate messages.

Thanks previously

!!!I don't want that somebody make it the code for me, just to advice me what i should do...

Can you find the minimum and maximum in a simple list of values? It's not really different from that.