Given a string that contains properly nested parentheses, return the decoded version of the string using the following rules:
- All characters inside each pair of parentheses should be reversed.
- Parentheses should be removed from the final result.
- If parentheses are nested, the innermost pair should be reversed first, and then its result should be included in the reversal of the outer pair.
- Assume all parentheses are evenly balanced and correctly nested.