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