swapping with out using third variable in c++

//in this programme 2 variables are entered thru keyboard, then a is assigned the sum of a & b, now b is assigned the sum i.e. a-b . it means now b has the value of a . now assign a as sum minus b i.e. a-b.

#include<conio.h>

#include<iostream.h>  

void main()

{

     int a,b;

    cout<<"enter a and b";

    cin>>a>>b;

    a=a+b;

    b=a-b;

    a=a-b;

    cout<<a;

    cout<<b;

    getch();

}