window.onkeydown = function (num) {
    if ( Screen_total.length < '15' ) {
        if ( keys[num.keyCode] == 'backspace' ) {
            Screen_total = Screen_total/10;
            Screen_total = Screen_total - (Screen_total%1);
            toScreen(Screen_total);
            console.log('Длина:' + Screen_total.length);
        } else {
            console.log(keys[num.keyCode] + ' (клавой)');
            if ( Screen_total != '' ) {
                Screen_total = Screen_total + keys[num.keyCode];
            } else {
                Screen_total = keys[num.keyCode];
            }
            console.log('Длина: ' + Screen_total.length);
            toScreen(Screen_total);
        }
    }
}