/in/foo.cc: In function 'int main()':
/in/foo.cc:14:12: warning: iteration 1000003 invokes undefined behavior [-Waggressive-loop-optimizations]
   if(!vis[i])
       ~~~~~^
/in/foo.cc:12:15: note: within this loop
  for(int i=2;i<=1000005;i++)
              ~^~~~~~~~~