C++ Solution

void countLetters(vector<int> &letters, string s, int begin, int end) {
    for (int i = begin; i < end; ++i)
        ++letters[s[i] - 'a'];
}

int numChanges(const string &s) {
    int length = s.length();
    if (length % 2) // odd
        return -1;
    int half = length / 2;
    vector<int> v1(26, 0), v2(26, 0), results(26, 0);
    countLetters(v1, s, 0, half);
    countLetters(v2, s, half, length);
    transform (v1.begin(), v1.end(), v2.begin(), results.begin(), [](int x, int y) { return abs(x - y); });
    int sum = accumulate(results.begin(), results.end(), 0);
    return sum / 2;
}