while (k <= n*m)
    {
        for (i = firstCol; i < lastCol; i++)
            a[firstRow][i] = v[k++];

        for (i = firstRow + 1; i < lastRow; i++)
            a[i][lastCol - 1] = v[k++];

        for (i = lastCol - 2; i >= firstCol; i--)
            a[lastRow - 1][i] = v[k++];

        for (i = lastRow - 2; i > firstRow; i--)
            a[i][firstCol] = v[k++];

        firstCol++;
        lastCol--;
        firstRow++;
        lastRow--;
    }