How to get Object to Background pixel ratio OBR) of an image

I am new in matlab,I am confused how to get value Object to background pixel ratio(OBR) of an image.This is my code.Please Help me.Thank you..

Image = imread('1.jpg');

BW = im2bw(Image);

[labeled,num] = bwlabel(BW,4);

nBlack = sum(BW(:));

nWhite = numel(BW) - nBlack;

OBR=num/nBlack;