how to add force in z direction in input.getmousebuttondown ?

hi, im using below script to add force to a ball, but it is adding force to X & Y axis only. Z is showing Zero. How to add force relative to swipe? I've attached this script to main camera so that i can wsipe at any point please halp me to z direction.

using UnityEngine; using System.Collections;

public class CameraScript : MonoBehaviour {

 Vector3 mousestart, mouseend;
 float starttime, endtime;
 Vector3 force;
 public GameObject ball;
 Rigidbody rb;


 // Use this for initialization
 void Start () 
 {
     rb = GetComponent<Rigidbody> ();
 }
 
 // Update is called once per frame
 void Update () 
 {
     if (Input.GetMouseButtonDown (0)) 
     {
         starttime = Time.time;
         mousestart = Input.mousePosition;
         
     }

     if (Input.GetMouseButtonUp (0)) 
     {
         endtime = Time.time;
         mouseend = Input.mousePosition;


         float totaltime = endtime - starttime;
         Vector3 distance = mouseend - mousestart;

         force = (distance/totaltime);
         Debug.Log(distance.z);
         ball.rigidbody.AddForce(force);
     }

 }

}