js著名的斐波那契數列 1 1 2 3 5 8，輸出第n項

     function  countNum(n){
            var first = 1,
                second = 1,
                third;
            if(n>2){
                for(var i = 0; i < n-2; i++){
                    third = first + second;
                    first = second;
                    second = third;
                }
                return third;
            }else{
                return 1;
            }
        }