#include <iostream>
#include "sales_item.hpp"

/*
 *
 * 1.5.2 example
 *
 *
 */

int main() {
	
	Sales_item item1, item2;
	std::cin >> item1 >> item2;
	if (item1.isbn() == item2.isbn()) {
		std::cout << item1 + item2 << std::endl;
		return 0;
	} else {
		std::cerr << "Data must refer to same ISBN"
				  << std::endl;
		return -1;
	}

	return 0;
}