void rankOfMatrix(int** A, int rows, int columns) {
    int rank = rows <= columns ? columns : rows;
    vector<char> line_used(rows);
    for (int i = 0; i < columns; ++i) {
        int j;
        for (j = 0; j < rows; ++j)
            if (!line_used[j] && A[j][i] != 0)
                break;
        if (j == rows)
            --rank;
        else {
            line_used[j] = true;
            for (int p = i + 1; p < columns; ++p)
                A[j][p] /= A[j][i];
            for (int k = 0; k < rows; ++k)
                if (k != j && A[j][i] != 0)
                    for (int p = i + 1; p < columns; ++p)
                        A[k][p] -= A[j][p] * A[k][i];
        }
    }
    cout << endl << "Ранг матрицы: " << rank << endl << endl;
}