[HackerRank] Diagonal Difference

int diagonalDifference(vector<vector<int>> arr) {
    int upper = 0, lower = arr.size()-1;
    int sum1 = 0, sum2 = 0;
    for (int i = 0; i < arr[0].size(); i++)
    {
        sum1 += arr[upper++][i];
        sum2 += arr[lower--][i];
    }
    return abs(sum1 - sum2);
}