13. (6 points) From a square with a side length of 5, cut off four small rectangles from the four corners, as shown in the figure. The perimeter of the resulting shape is $\qquad$ .
【Analysis】According to the figure, after cutting off four small rectangles from the four corners of the large square, although the area is reduced, its perimeter remains unchanged. Therefore, the perimeter formula of the square can be used to solve the problem.

【Solution】Solution: $5 \times 4=20$,
Answer: The perimeter of the cut shape is 20.
So the answer is: 20.
【Comment】The key to solving this problem is to understand: after cutting off four small rectangles from the four corners of the large square, although the area is reduced, its perimeter remains unchanged.