What method should I use instead for LostFocus for the Page in PageFrame?

In the Page1 I have some text boxes , the first one is a sum of other two and when I go by arrows or by clicking the mouse on Page2 – I want to check the text box1 Value whether it is equal to sum of others or not. If it is not – I don’t want to lose focus of Page1 and want to return focus on the first text box in Page1.
But PageFrame doesn’t have method LostFocus, so every time when I click with mouse or move by arrows to the Page2, it receives focus and I can not return it to the Page1.

Please help!!!
GAlia