How to get back to the nearest equal original image from the exposure image.?

I am working to normalize images from over and under exposure.

I=imread('cameraman.tif');
imshow(I);
[r,c]=size(I);
avg=sum(sum(I))/(r*c);
Imask=-avg;
U=I+Imask;
I1mask=avg;
O=I+I1mask;
imwrite(O,"over.png");
imwrite(U,"under.png");

From this Over and Under exposure images, i wanted to get back to the nearest original image without taking original image into reference.

I tried these steps,

grayImage=imread('over.png');
I=imread('cameraman.tif');
normalizedImage = uint8(255*mat2gray(grayImage));
figure
imshow(I);
figure
imshow(grayImage);
figure
imshow(normalizedImage);

But still I need a better image. Is that possible? Can you help me.