Problem with Animation

I put this script and the animation doesnt work well:

var AnimName ="DartAnimation";

private var resetTime = 0.0000000000000000;

var useBackwards = true;

function Update (){
    if(useBackwards==true){
        if(Input.GetKeyDown(KeyCode.LeftAlt)){
            animation[AnimName].speed = 1; 
            animation[AnimName].time = resetTime;
            animation.Play(AnimName);
        }
        if(Input.GetKeyUp(KeyCode.LeftAlt)){
            animation[AnimName].speed = -1; 
            resetTime =animation[AnimName].time;
            animation[AnimName].time = animation[AnimName].length;
            animation.Play(AnimName);
        }
    }
}

and it doesnt work right

ITS FIXED