/in/foo.cc: In function 'void shaizi()':
/in/foo.cc:18:12: warning: iteration 100048 invokes undefined behavior [-Waggressive-loop-optimizations]
   if(!vis[i])
       ~~~~~^
/in/foo.cc:16:15: note: within this loop
  for(int i=2;i<=N;i++)
              ~^~~