How can I improve time complexity in this function from Codility?

Like in the title. I have this function that returns a number which occured odd times in the array. And I need to make it at least 0.28s faster :) . What can i do to improve this programme?

int OddOccurencesInArray(vector<int> &A) {
    map<int, size_t> mapa;

    for (int& i : A){
        mapa[i]++;
    }

    for (auto& p : mapa){
        if(p.second % 2 == 1){
            return p.first;
        }
    }
    return -1;
}