#include <iostream>

struct volume
{
    int lit ;
    int ml ;
};

volume operator+ ( volume a, volume b )
{
    const int ml = a.ml + b.ml ;
    return { a.lit + b.lit + ml/1000, ml%1000 } ;
}

std::ostream& operator<< ( std::ostream& stm, volume vol )
{ return stm << "( " << vol.lit << "L " << vol.ml << "ml )" ; }


int main()
{
      volume a { 12, 650 } ;
      volume b { 29, 723 } ;
      std::cout << a << " + " << b << " == " << a+b << '\n' ;
}