attempt to execute rgb2gray but showing error

Here's my code

clc;                       %rgb2gray
a=imread('2.jpg');  
c=rgb2gray(a);
f=double(c);
[row, col, dim]=size(f);
b=255;
g=b-c;
subplot(2,2,1);
imshow(a);
title('original image');
subplot(2,2,2);
imshow(c);
title('rgb2gray');
subplot(2,2,3);
colormap(gray);
imshow(g);
title('negative image');

The error message is :

Error in rg (line 3)

c=rgb2gray(a);

[Steven Lord]

That's not the full text of the error message. Show us all the text displayed in red.

Answer 1

[Image Analyst]

Replace this:

a=imread('2.jpg');  
c=rgb2gray(a);
f=double(c);
[row, col, dim]=size(f);

with this:

a = imread('2.jpg');  
[row, col, numberOfColorChannels] = size(a)
if numberOfColorChannels == 3
    % Convert only if color
    grayImage = rgb2gray(a);
else
    grayImage = a;
end
f=double(grayImage);