Even-tual Reduction

You are given a string $S$ with length $N$. You may perform the following operation any number of times: choose a non-empty substring of $S$ (possibly the whole string $S$) such that each character occurs an even number of times **in this substring** and erase this substring from $S$. (The parts of $S$ before and after the erased substring are concatenated and the next operation is performed on th