square image with circle in the middle, shortest answer in matlab

main.m
img = imread('image.jpg');
[rows, cols, ~] = size(img);
[r, c] = ndgrid(1:rows, 1:cols);
mask = sqrt((r - rows/2).^2 + (c - cols/2).^2) <= min(rows,cols)/2;
img(~mask) = 255;
imshow(img);
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