var setBackgroundStyle = function($arg){
	var len = $arg.length;
	var str = ["si1","si2"];
	for(var i = 0;i < len;i++){
		var a = i % 2;
		var b = Math.floor(i / 2) % 2;
		$($arg[i]).addClass(str[(a + b) % 2]);
	}
}