Something

#include <cstdlib>
#include <iostream>

using namespace std;

long add, sub, mainvar, addvar, subvar;
int choice;


long addition(long a){
              mainvar = mainvar + a;
              return (mainvar);
              }
              
long subtraction(long a){
                 mainvar = mainvar - a;
                 return (mainvar);
                 }
                 
                 
int main(){
    cout << "Press 1 to add or 2 to subtract from " << mainvar << "\n \n \n";
    cin >> choice;
        system("CLS");
    
    if(choice == 1){
              cout << "Insert number to add to " << mainvar << ": ";
                cin >> addvar;
                addition(addvar);
                }
    else if(choice == 2){
                  cout << "Insert number to subtract from " << mainvar;
                cin >> subvar;
                addition(subvar);
                }
    else
           case 1:
                
           case 2:               
                
           default:
                   cout<< "lol wut? You entered a non-choice";
                   }
    system("PAUSE"); 

}