How to save an image into direct folder with increment filename using imwrite()?

i'm new in matlab and i have a problem that i can't solve it like this. Here my new code :

    folder = 'H:\SKRIPSI\Citra Latih 2\';
    ImageFiles = dir(folder);
    a = length(ImageFiles)-1;
    for Index = 1:a
    baseFileName = [num2str(Index),'.jpg'];
    imwrite(img.cdata,fullfile(folder,baseFileName));
    end

the code above can save images in increment filename like 1.jpg, 2.jpg, 3.jpg and so on but still overwrite the previous images with the new images. What is worng with my code? can you help me to fix it?

[Stephen23]

Duplicate:

http://www.mathworks.com/matlabcentral/answers/289707-how-to-prevent-image-that-has-been-stored-not-overwrite-using-imwrite