C++ problem about finding min and max

[n_angelov]

Hello everybody :)
I've to solve C++ problem and there it is:
You have to write a program in C++ that computes the difference between the maximal and the minimal 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 the minimal or maximal values do not exists, your program should output appropriate messages.

Actually I have written a part of it:

#include <iostream>
#include <cmath>
using namespace std;

int main()
{
    double a,b,c,d,x,y,f;
    
    cout<<"Please add four numbers each of them corresponds to a, b, c and d."<<"\n";
    cout<<"A=";
    cin>>a;
    
    cout<<"\n"<<"B=";
    cin>>b;
    
    cout<<"\n"<<"C=";
    cin>>c;
    
    cout<<"\n"<<"D=";
    cin>>d;
    
      for ( x=a; x<=b; x=x+0.01)
        {
            for ( y=c; y<=d; y=y+0.01)
    
            f=sin(y)-cos(x*x);
            
            cout<<"Answer: "<<f<<"\n";

                                             }

                                 
        system ("pause");
        return 0;
}

...So I want to take min and max of f . I've tried some things but my knowledge of C++ is not so big.

Email: <<removed>>

Thank you.