function Im=sumImages(fileIndex,theDir);
%function Im=sumImages(fileIndex,theDir);
%adds tif files in fileIndex, under theDir, if no directory assumes current
%doesn't use filenumbers, just the order in which they appear in the
%directory
%minus sign subtracts

if nargin == 1
    theDir = '.'
end

ImFiles =dir(strcat(theDir,'/*.tif'));
Im = zeros(195,487);

for j=fileIndex
    if (j<0)
        j=-j; %subtract file
        s=-1;
    else
        s=1;  %add file
    end
    fprintf('adding file %s\n',ImFiles(j).name);
    Im=Im+s*imageread(strcat(theDir,'/',ImFiles(j).name),'tif',[487,195],32);
end