I cant move more than 3 divs over. How can I get it to slide if I add more divs??

Code:
////////////////////////////////
// jQuery Main Slide Function //
////////////////////////////////

var CURRPOS = 0;

$(document).ready(function() {

	$(".nextBtn").click(function(){
		if(CURRPOS == 0 || CURRPOS == -900)
		{
			CURRPOS -= 900;	
			$("#movePage").animate({
				marginLeft: CURRPOS+"px"
			},500);
			return;
		}
		if(CURRPOS == -1800)
		{
			CURRPOS = 0;	
			$("#movePage").animate({
				marginLeft: CURRPOS+"px"
			},500);
		}
	});
	
	$(".backBtn").click(function(){
		
		if(CURRPOS == -900 || CURRPOS == -1800)
		{
			CURRPOS += 900;	
			$("#movePage").animate({
				marginLeft: CURRPOS+"px"
			},500);
		}
		
	});
});

////////////////////////////////////
// End jQuery Main Slide Function //
////////////////////////////////////
Thanks in advance!!