get pixel value by(x,y) simultan with vs2008 c#

public static bool Invert(Bitmap b)
{
    // GDI+ still lies to us - the return format is BGR, NOT RGB. 

    BitmapData bmData = b.LockBits(new Rectangle(0, 0, b.Width, b.Height), 
    ImageLockMode.ReadWrite, PixelFormat.Format24bppRgb); 
    int stride = bmData.Stride; 
    System.IntPtr Scan0 = bmData.Scan0; 
    unsafe 
    { 
        byte * p = (byte *)(void *)Scan0;
        int nOffset = stride - b.Width*3; 
        int nWidth = b.Width * 3;
        for(int y=0;y < b.Height;++y)
        {
            for(int x=0; x < nWidth; ++x )
            {                                       
                p[0] = (byte)(255-p[0]);    // Here I want to get by pixel(x,y)
                ++p;                        // position simultaneously for exp:
                                            // result= 
                                            // pixel(0,0)+pixel(1,0)+pixel(2,0)
                                            // +pixel(0,1)+pixel(1,1)+ pixel(2,1)
                                            // +pixel(0,2)+pixel(1,2)+ pixel(2,2)
                                            // result=result/9;
                                            

            }           
            p += nOffset;                   //for what this function "p+=nOffset"?
        }                                   //I try to disable it and still work
    }
    b.UnlockBits(bmData);
    return true;
}