// you can also use includes for example:
// #include <algorithm>

unsigned long long sum(const vector<int> v) {
    unsigned long long total = 0;
    for(int i = 0; i < v.size(); ++i) {
        total += v[i];
    }

    return total;
}

int equi ( const vector<int> &A ) {
    int size = A.size();
    unsigned long long front = 0, back = sum(A);

    for(int i = 0; i < size; ++i) {


        back -= A[i];
        if(front == back) return i;


        front += A[i];

    }

    return -1;
}