private Vector3 firstPoint;
    private Vector3 secondPoint;

    void Update() {

        if(Input.GetMouseButtonDown(0)) {
            firstPoint = Input.mousePosition;
        }

        if(Input.GetMouseButton(0)) {
            secondPoint = Input.mousePosition;
        }

        if(Input.GetMouseButtonUp(0)) {
            Rect screenRectAngle = new Rect(firstPoint.x, firstPoint.y, secondPoint.x - firstPoint.x, secondPoint.y - firstPoint.y);
            foreach(GameObject unit in GameObject.FindGameObjectsWithTag("Unit")) {
                Vector3 screenSpacePos = Camera.main.WorldToScreenPoint(unit.transform.position);

                if(screenRectAngle.Contains(screenSpacePos)) {
                    unitsSelected.Add(unit.GetComponent<UnitController>());
                    print(unit.name + " selected.");
                }
            }
        }