that returns the euclidean distance between two points a and b.
You only need to submit the required classes;
your main program will be ignored.
Strictly obey the type definitions of the statement.
The official statement of a problem is always the one
in the PDF document. The HTML and PNG versions of the statement
are also given to help you, but they may contain some content
that is not well displayed. In case of doubt, always use the PDF.