Unity 3d 如何实现点击屏幕，npc自动移动到点击位置

using UnityEngine;
using System.Collections;


public class Player : MonoBehaviour
{
    private Vector3 clickPosion;
    public float speed = 5f;


    void Start()
    {
        clickPosion = transform.position;
    }
    void Update()
    {
        if(Input.GetMouseButtonDown(0))
        {
            Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);
            RaycastHit hit;
            Physics.Raycast(ray, out hit);
            try
            {
                if (hit.collider.tag == "Ground") &n