Simulating Undo in a Text Editor

You are given a series of operations to simulate a simple text editor with undo functionality. The editor starts with an empty text. It supports the following operations:

• add <char>: Append the character char to the end of the current text.
• delete: Remove the last character from the text (if any).
• undo: Undo the most recent add or delete operation that has not yet been undone.

The undo operation reverses the effect of the last non-undone operation. For an add operation, it removes the appended character; for a delete operation, it restores the deleted character. Formally, if the last operation was an add of character c, then undo removes that c; if it was a delete of character c, then undo appends c back to the text.

In LaTeX, the undo effect of an operation can be described as follows:

\( \text{undo}( \text{add}(c)) = \text{delete}(c) \) and \( \text{undo}( \text{delete}(c)) = \text{add}(c) \).

Your task is to simulate these operations and output the final text after processing all operations.

inputFormat

The input is read from standard input (stdin) and is structured as follows:

1. The first line contains an integer n representing the number of operations.
2. The next n lines each contain a string representing an operation. Each operation is one of the following forms: add <char>, delete, or undo.

outputFormat

Output to standard output (stdout) a single line that is the final text after all operations are processed.

## sample

6
add a
add b
add c
delete
undo
undo

ab