Using OMP parallel to add to count in for loop

Question

I'm trying to speed up some processing with OMP parallel

but the cnt value is different every time, it's not adding across all for some reason. thanks.

        RGBTRIPLE rgb;
        COLORREF color;
        int cnt = 0;

#pragma omp parallel for private(color, rgb) shared(cnt)
        for(short ii=0;ii<cx;ii = ii + 1) 
        {
            for(short j=0;j<cy;j = j + 1)
            {
                color = pDC->GetPixel(ii,j);


                rgb.rgbtRed = GetRValue(color);
                rgb.rgbtGreen = GetGValue(color);
                rgb.rgbtBlue = GetBValue(color);
                if(rgb.rgbtRed>0 && rgb.rgbtRed<255) 
                {
#pragma omp critical
                    cnt++;
                    //TRACE("color: %d,%d,%d\n",rgb.rgbtRed,rgb.rgbtGreen,rgb.rgbtBlue);
                }
            }
        }



#pragma omp single
        {}

        CString msg; msg.Format("%d done",cnt);
        AfxMessageBox(msg);
    }