How to parse information from selenium c# element code?

Question

C++ / C#

There is a site with captcha, you need to enter in the input how many stars in the picture. However, I noticed a very interesting solution.
In the element code in the picture name there are figures 10.jpg, 13.jpg.
For example, a picture appeared, I parse information from the code of the element name of the picture, for example 13.jpg, take the name and insert the number 13 into the input
How can this be implemented? And is it feasible at all?