Show sum of user into label

Hello 
I have two dropdownlists one of the country (worlddrdolist) and the another is of states (staatddl) and the button (NumOfUsrBtn) where if the user since click on it then the label will show the total sum of users depending on a country selected from (worlddrdolist) First whatever if the user select the state or not also if the user select the state then it will show the users depending of the country , I hope if some one can help me with writing the code of this case. And many thanks