/in/foo.cc: In function 'void init()':
/in/foo.cc:15:12: warning: iteration 100005 invokes undefined behavior [-Waggressive-loop-optimizations]
   if(!vis[i])
       ~~~~~^
/in/foo.cc:13:11: note: within this loop
  for(i=2;i<=N;++i)
          ~^~~