I was upsolving some problems from the Open Cup GP of ICL 2013. When I wanted to submit problem F (Cost of the Question), its name in the submission window included "NOT_SOLVABLE". However, I got Accepted. So can anyone who has participated in that contest please clarify what does "NOT_SOLVABLE" mean? Was there a technical problem with this task's grader at some point? Or is the problem literally not solvable conceptually and the judge solution is wrong? The problem seems to have been solved during the contest and it's still in the standings.