How to Move an Object On Y Axis with Mouse in Unity - unity3d

Question

I want control the transform with mouse just for Y axis as it is moving on X axis by it self.
I use Vector2.MoveTowards to move it on X axis.
The transform must be moved by any touch and drag on the screen. So I wrote my codes in the Update function
My Codes :
using UnityEngine;
using System.Collections;
public class PlayerMovement : MonoBehaviour
{
public Camera playerCamera;
public float speed = 12.0F;
private Vector3 moveDirection = Vector3.zero,Point,last;
void Start()
{
Point = transform.position;
last = transform.position;
if (playerCamera == null)
{
playerCamera = Camera.main;
}
playerCamera.transparencySortMode = TransparencySortMode.Orthographic;
}
void Update(){
if(Input.GetMouseButtonUp(0))
Point = last;
moveDirection.x = transform.position.x+speed;
moveDirection.y = transform.position.y;
if (Input.GetMouseButtonDown(0)) {
Ray ray = Camera.main.ScreenPointToRay (Input.mousePosition);
last = ray.GetPoint (0);
transform.position = new Vector2 (transform.position.x, last.y - Point.y);
}
else
transform.position = Vector2.MoveTowards( transform.position,moveDirection,speed*Time.deltaTime);
//moveDirection.y -= 20.0f * Time.smoothDeltaTime;
//After we move, adjust the camera to follow the player
playerCamera.transform.position = new Vector3(transform.position.x, playerCamera.transform.position.y , playerCamera.transform.position.z);
}
}
How can I solve it?