final cleanup move inputMark into loop.
Test using exercise example:
$ java E02DoWhile
Input a mark: 13
Input a mark: 45
Input a mark: 63
Input a mark: 73
Input a mark: 101
Input a mark: 45
Input a mark: 18
Input a mark: 92
Input a mark: -1
There are 7 students: 2 distinctions, 1 pass, 4 fails (plus 1 invalid)