rocket instantiate fire 2 problem

i want to shoot my projectile when. and only when i right click. this script has it when i click both and i cant seem to fix it. please help

var projectile : Rigidbody;
var initialSpeed = 20.0;
var reloadTime = 0.5;
var ammoCount = 20;
private var lastShot = -10.0;
function Update()
{
if (Input.GetMouseButtonDown(1))  
   {     Fire ();
   }
}

function Fire () {
    // Did the time exceed the reload time?
    if (Time.time > reloadTime + lastShot && ammoCount > 0) {
        // create a new projectile, use the same position and rotation as the Launcher.
        var instantiatedProjectile : Rigidbody = Instantiate (projectile, transform.position, transform.rotation);

        // Give it an initial forward velocity. The direction is along the z-axis of the missile launcher's transform.
        instantiatedProjectile.velocity = transform.TransformDirection(Vector3 (0, 0, initialSpeed));

        // Ignore collisions between the missile and the character controller
        Physics.IgnoreCollision(instantiatedProjectile.collider, transform.root.collider);

        lastShot = Time.time;
        ammoCount--;
    }
}

When you say "when I click both" are you saying that you're hitting both left and right mouse buttons at the same time and it's still firing, or that you're hitting the left mouse button only and it's still firing?

If it's the former, you could do something like seeing if the mouse button 0 is NOT down in your check to see if the mouse button 1 is.