为什么加入if判断后，图片切不了了啊

Question

var list = document.getElementById('list');
var prev = document.getElementById('prev');
var next = document.getElementById('next');
function animate (offset) {
         var newLeft =  parseInt(list.style.left) + offset;
         list.style.left = newLeft+'px';
         if(newLeft>-1000){
             list.style.left=–3000+'px';
         }
         if(newLeft<-3000){
             list.style.left=–1000+'px';
         }
     }
     next.onclick = function () {
         animate(-1000);
        }
    prev.onclick = function () {
        animate(1000);
    }