Undo Action Tracker

You are implementing an undo feature for a simple text editor. Each action can be either 'ADD X' (adding character X) or 'UNDO' (removing the last action). Write a program that processes a sequence of actions and outputs the final state of the text. If there are no actions, output an empty string. Input format: - Each action is provided on a new line, and the last input line will be 'END'. Output format: - A single line containing the final text after all actions. Example: Input: ADD a ADD b UNDO ADD c END Output: a c