int number_needed(string a, string b) {
    auto count = 0;
    vector<int> freq(26, 0);
    for (auto c : a) { ++freq[c - 'a']; }
    for (auto c : b) { --freq[c - 'a']; }
    for (auto val : freq) { count += abs(val); }
    return count;
}