unity 2d Drag object

 private Vector2 screenPoint;
    private Vector3 offset;

    void OnMouseDown() {

        offset = gameObject.transform.position - Camera.main.ScreenToWorldPoint(new Vector2(Input.mousePosition.x, Input.mousePosition.y));
    }

    void OnMouseDrag()
    {
        Vector2 curScreenPoint = new Vector2(Input.mousePosition.x, Input.mousePosition.y);
        Vector2 curPosition = Camera.main.ScreenToWorldPoint(curScreenPoint) + offset;
        transform.position = curPosition;
    }