How can I compute the intensity of contour of an image?

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Hello guys, if I have an image with 11x11 pixels and in the center of the image is a square of 5x5 pixels, with the gray level of the background 0 and the gray level of the square is 50. How can I compute the result of the magnitude of edges(intensity of the contour), or better said, the gradient of the image, given by the compass operator for this image taking into account that the image is not noisy? I need to apply simple derivation because the image it's not noisy.