/in/foo.cc: In function 'int main()':
/in/foo.cc:69:8: warning: iteration 500008 invokes undefined behavior [-Waggressive-loop-optimizations]
   lg[i]=lg[i/2]+1;
   ~~~~~^~~~~~~~~~
/in/foo.cc:68:15: note: within this loop
  for(int i=2;i<=maxn;i++)
              ~^~~~~~