histogram equalization in for boucle

please i want to equaliz histogram of an 40 images (.bmp) and save them in the same folder, and i wrote this code but i don't know the error

for i=1:40 a = imread(i,'.bmp'); b = histeq(a); imwrite(b,'.bmp'); end

it gives me this error Error using fopen Invalid filename.

Error in imread (line 343) fid = fopen(filename, 'r');

Error in aqqq (line 2) a = imread(i,'.bmp');