Very strange... I tried all tests here and they all worked well. It's a DP,right? But I always get wa4. I added this to my code: when I worked out my answer, I'll check them myself. If there are some shouldn't be in the same pair,then while(1). As a result I got TLE4. So it obviously I've made a wrong answer. BUT HOW COULD IT HAPPEN!!!

Well, it was because of my wrong algo.
And I've got WA7 later, that's because I misspelled IMPOSSIBLE in dfs. (There were two of this word in my prog, one is in dfs and the other is after DP)
It took me an hour to debug for this! Whoa-a!