/in/foo.cc: In function 'int main()':
/in/foo.cc:10:41: warning: operation on 'i' may be undefined [-Wsequence-point]
   10 |         for(int i=0;i<3;) cin>>x[i]>>y[i++]; // 紧凑输入
      |                                        ~^~
/in/foo.cc:22:18: warning: variable 'b' set but not used [-Wunused-but-set-variable]
   22 |         int a=-1,b,c;
      |                  ^