/in/foo.cc:6:6: warning: ISO C++ forbids declaration of 'main' with no type [-Wreturn-type]
 main()
      ^
/in/foo.cc: In function 'int main()':
/in/foo.cc:9:24: warning: unused variable 'x' [-Wunused-variable]
   for(register int i=0,x;i<n;++i)scanf("%d%d",a+i,b+i),c[a[i]].push_back(i);
                        ^