Want to find Entropy between two approximation part of Image, (Approximation part got from DWT of an Image)

I want to find entropy between two approximation part of image, Approximation part got from performing DWT on an Image. I have used e = sum(CA*log(CA./CA1)); where CA & CA1 are approximation part of Image 1 & Image 2 respectively. From above formula i got e as (1*256). CA & CA1 both are (256*256) part. I want e as (256*256).