var lastScrollTop = 0;
$(window).scroll(function(event){
   var st = $(this).scrollTop();
   var slide = 1
   if (st > lastScrollTop){
    if(checkBounds(slide)) {
        slide++;
       $('html, body').animate({
        scrollTop: $(".slide-" + slide).offset().top

    }, 1200);
   } else {
    slide = slide;
   }
   } else {

        if (checkBounds(slide)) {
     slide--;
     $('html, body').animate({
        scrollTop: $(".slide-" + slide).offset().top

    }, 1200);
 } else {
    slide = slide;
 }
   }
   lastScrollTop = st;
});

function checkBounds(slide) {
    return slide < 3 && slide >= 1;
}