why does my program does not do proper diagonal difference(C++)?

for some reason my program does not give me the correct diagonal difference.for example in the input of (11 2 4) (4 5 6) (10 8 -12) its supposed to give me 15 by adding (11+5+(-12)) and (4+5+10) then doing abs difference of two, but gives me 6 for some reason.

Here is my function for the code:

int diagonalDifference(vector<vector<int>> arr) {
       int result = 0;
       int difference = 0;
       int first_diagonal =0;
       int second_diagonal = arr.size();
       int add_for_first_diagonal = 0;
       int add_for_second_diagonal = 0;
       for(int row = 0; row < (int)arr.size(); row++) {
           for(int column =0; column < (int)arr.size(); column++) {
               add_for_first_diagonal += arr[column][first_diagonal]; 
               add_for_second_diagonal += arr[column][second_diagonal];
        }
          first_diagonal += row;
          second_diagonal -= row;
    }
      difference = first_diagonal - second_diagonal;
      result = abs(difference);
      return result;
}

Any help is appreciated. Thank you.