Getting (Input) values before running rest of script.

Hi! I am trying to create something like a "drag the direction to shoot". I have this to get Input coords:

         void Update ()
 {
         Vector3 Point1;
         Vector3 Point2;
         Vector3 Point3;
         if (Input.GetKeyDown("space"))
     {
         Point1 = Camera.main.ScreenToWorldPoint( new Vector3(Input.mousePosition.x, Input.mousePosition.y,10));
         }
     if (Input.GetKeyUp("space"))
     {
         Point2 = Camera.main.ScreenToWorldPoint( new Vector3(Input.mousePosition.x, Input.mousePosition.y,10));
         }
     if (Point1.z ==10 || Point2.z ==10)
 {
         
     Point3.y = 20;
     if (Point1.x == Point2.x)
         {
             Point3.x = Point2.x;
         }
     else
         {
             float mouse_grad =(Point1.y-Point2.y)/(Point1.x-Point2.x);
             Point3.x=((Point3.y-Point2.y)+(mouse_grad*Point2.x))/mouse_grad;
         }
         
          transform.position = new Vector3(Point3.x, Point3.y, 10);
                 
 }
             
 }

Although logically this SHOULD work, Unity won`t let me use it because of use of possibly unassigned values (After the third if). Maybe anyone of you knows how to bypass this with a different loop?

I know this is actually a logic problem, not scripting, but I have been thinking about it since the last evening, and I can not seem to figure out a solution for it to work. (Meanwhile I am a a noob, and do not have a long experience with scripting, so if the solution is ridiculousl easy please at least point me in the direction guys :) )

Have a great day!