bgc = new Array();
bgc[0] = "url(img/top/top1.jpg)";
bgc[1] = "url(img/top/top4.jpg)";
bgc[2] = "url(img/top/top2.jpg)";
bgc[3] = "url(img/top/top3.jpg)";
n = Math.floor(Math.random()*bgc.length);
document.getElementById('contents').style.backgroundImage=bgc[n];