var counter = 0;

$('.strell_left').click(function(){

    var slides = $('.slider-item');
    if (counter <= (slides.length-2) && counter >= 0 ) {
        counter--;

        $('.slider').animate({
            'right': +(counter * 100) + '%'
        }, 1000);

    }
    else {
     {
            counter = slides.length-1;
            $('.slider').animate({
                'left': -(counter * 100) + '%'
            }, 1000);
        }
    }

});