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

// you can write to stdout for debugging purposes, e.g.
// cout << "this is a debug message" << endl;

int solution(vector<int> &A) {
    // write your code in C++14 (g++ 6.2.0)
    signed long long sumLow = 0;
    signed long long sumHigh = 0;


    // calculate sum of higher elements for P=0
    for ( unsigned int i = 1 ; i < A.size() ; i++ )
    {
        sumHigh += A[i];
    }

    // check indices until we find an equilibrum index
    for ( unsigned int p = 0 ; p < A.size() ; p++ )
    {
        if ( sumLow == sumHigh )
        {
            return p;
        }

        if ( p+1 < A.size() )
        {
            // prepare data for next cycle
            sumLow += A[p];
            sumHigh -= A[p+1];
        }
    }

    return -1;
}