QT getting the screen resolution without the extended monitor

QDesktopWidget desktop;

int desktopHeight=desktop.geometry().height();
int desktopWidth=desktop.geometry().width();
this->resize(desktopWidth,desktopHeight);

for example my main monitor resolution is 1024 x 768 and my extended monitor resolution is 800x600. the value i get from desktop height and width is the sum of the two monitor resolution. i only need to get the main monitor resolution. can somebody tell me how? thanks. I'm using qt 4.7.3

edit: I'm using dual screen, particularly in cashier ordering the main monitor is seen by the cashier and the extended are seen by the costumer.