RadGrid Row Click or Selected Event

[Mmm]

I have a RadGrid and I am filtering that grid using a text box, I just want to disable the textbox if more than one row is selected and if no rows or only one row is selected then I don't want the text box to be disabled. Rows are selected using checkbox or just clicking on it.

I am not sure how do I do this as there is no row click even for rad grid

Please help

Thanks