code: public String buildFilename(final ProblemInput input, final int attempt) {
  		final StringBuilder builder = new StringBuilder();
  		final Problem problem = input.getProblem();
  		final ContestInfo info = problem.getParent();
  		final int index = info.getProblems().indexOf(problem);
  		final char letter = (char) ((int) 'A' + index);
  		builder.append(letter)
  			.append(FILENAME_SEPARATOR)
  			.append(input.getName())
  			.append(FILENAME_SEPARATOR);
  		if (attempt == -1) {
  			builder.append(PRACTICE);
  		}
  		else {
  			builder.append(attempt);
  		}
  		builder.append(INPUT_EXTENSION);
  		return builder.toString();
  	}
docs: <p>Builds and returns a valid file name
  for the given problem <tt>input</tt>.</p>
  
  @param input Input to retrieve file name from.
  @param attempt Attempt number.
  @return Built file name.