Images can't be saved in bmp format

Hi,
I've tried to save images in bmp format, my code worked but I didn't get bmp images (ROI-1.bmp ... ROI-8.bmp). I just got ROI-1 ... ROI-8. Could you help me to solve this problem? Thanks
Here is my code

for i = 1 : 8
    str = int2str(i);
    str1 = strcat('D:\gbr_nn\bmp\ori_',str,'.bmp');
   [im, map] = imread(str1);
   ROI = imcrop(im, [50 100 400 300]);
  cd('D:/gbr_nn/bmp/cropbmp');
   x = strcat('roi-',str);
  imwrite(ROI, x, 'bmp');
end