[рони]

window.addEventListener("DOMContentLoaded", function() {
    var a = document.querySelectorAll(".button-class-material"),
        b = document.querySelector(".demo");
    [].forEach.call(a, function(a, c) {
        var d = ("0" + (c + 1)).substr(-2);
        a.addEventListener("click", function() {
            b.style.backgroundImage = "url(http://l-c.by/d/402010/d/32_ays_" + d + ".jpg)"
        }, !1)
    })
});