#pragma omp parallel for reduction(+:sum)
    for (int k = 0; k < COLUMNS; k++) {
        for (int i = 0; i < ROWS; i++) {
            int j = 0;
            for ( j; j < COLUMNS; j++) {
                sum += matrix_a[i][k] * matrix_b[k][j];
            }
            matrix_r[i][j] = sum;
        }
    }