html

<select class="dice-sides-input">
                <option></option>
                <option>1</option>
                <option>2</option>
                <option>3</option>
                <option>4</option>
                <option>5</option>
                <option>6</option>
                <option>22</option>
            </select>

javascript

var getNum = function () {
	sides = $('.dice-sides-input :selected').text();
	return Math.floor(Math.random() * sides) + 1
}