Drag object in unity 2d with moving camera

I want to drag my 2d object in unity2d using code with:

 void OnMouseDown()

 {

 screenPoint = Camera.main.WorldToScreenPoint(gameObject.transform.position);
 offset = gameObject.transform.position + Camera.main.ScreenToWorldPoint(new Vector3(Input.mousePosition.x, Input.mousePosition.y, screenPoint.z));
 }

 void OnMouseDrag()
 { 
 Vector3 curScreenPoint = new Vector3(screenPoint.x, Input.mousePosition.y, screenPoint.z);

 Vector3 curPosition = Camera.main.ScreenToWorldPoint (curScreenPoint);
 transform.position = curPosition;

 }

Problem is that my camera is also moving along x axis so when I click on gameobject to drag it, it moves along x axis. I want to make this gameobejct static so it will be in its place and won't move? Any suggestions?