I try to fail many times. 
   So I contacted the PKU administrator and told him about my condition. 
After a while, he told me just one word:"Getchar is never a good choice."
   You can guess the result.
   I use scanf instead and get AC.
   The former time I get TLE because when editing the input file the administrator destroys the correct format, so that the getchar doesn't work and the program enters dead loop. 
   Everything comes to end.