function solve() {
    let oldBooks = document.querySelectorAll('#outputs .bookShelf')[0];
    let newBooks = document.querySelectorAll('#outputs .bookShelf')[1];
    document.querySelector('body form button').addEventListener('click', addBook);
    function addBook(e) {
        e.preventDefault();
        const fields = document.querySelectorAll('body form input');
        const title = fields[0].value;
        const year = Number(fields[1].value);
        let price = Number(fields[2].value);
        //triple condition
        if(title === '' || year < 1 || price <= 0) { return; }

        const book = createBook(title,year,price);
        year >= 2000 ? newBooks.appendChild(book) : oldBooks.appendChild(book);
    }

    function createBook(title, year, price) {
        let divBook = document.createElement('div');
        divBook.className = 'book';

        let paragraphBook = document.createElement('p');
        paragraphBook.textContent = `${title} [${year}]`;
        divBook.appendChild(paragraphBook);

        year < 2000 ? price = discount(price) : price;
        let buttonBuyBook = addButton(`Buy it only for ${price.toFixed(2)} BGN`);
        buttonBuyBook.addEventListener('click', buyBook);
        divBook.appendChild(buttonBuyBook);
        
        if(year >= 2000) {
            let buttonOldSection = addButton('Move to old section');
            buttonOldSection.addEventListener('click', toOldSection);
            divBook.appendChild(buttonOldSection);
        }
       
        return divBook;
    }

    function discount(price) {
        return price * 0.85;
    }

    function addButton(context) {
        let button = document.createElement('button');
        button.textContent = context;
        return button;
    }

    function buyBook(e) {
        let storeProfit = document.querySelectorAll('body h1')[1];
        let oldProfit = Number(storeProfit.textContent.split(' ')[3]);

        let bookPrice = Number(this.textContent.split(' ')[4]);
        storeProfit.textContent = `Total Store Profit: ${(oldProfit + bookPrice).toFixed(2)} BGN`;
        this.parentNode.remove();
    }

    function toOldSection(e) {
        let bookMoved = this.parentNode;
        //remove only button to old sectio!!!
        this.remove();

        let buyBook = bookMoved.querySelector('button');
        let oldPrice = Number(buyBook.textContent.split(' ')[4]);
        buyBook.textContent = `Buy it only for ${discount(oldPrice).toFixed(2)} BGN`;
        oldBooks.appendChild(bookMoved);
    }
}