Cannot implicitly convert type 'int' to 'byte' [on hold]

how can I fix this code?

byte b = (byte)((double)array[(int)num5 * bitmapData.Stride + num7 + 2] 
         * 0.299 + (double)array[(int)num5 * bitmapData.Stride + num7 + 1] 
         * 0.587 + (double)array[(int)num5 * bitmapData.Stride + num7] 
         * 0.114);

if (reverse)
{
    b = 255 - b;

The error is in the "255 - b;"

Thanks in advance Gino