Platformer character movement

Hello what im trying to do is create a mario style platformer but im alittle stuck on the movement of the character. How would i alter this script so that the character can only walk left and right with the walk animation

 function Update ()
 {
    if (Input.GetAxis("Vertical") > 0.2)
        animation.CrossFade ("walk");
    else
       animation.CrossFade ("idle");
 }