/in/foo.cc: In function 'int main()':
/in/foo.cc:29:8: warning: iteration 2000004 invokes undefined behavior [-Waggressive-loop-optimizations]
   fa[i]=i;
   ~~~~~^~
/in/foo.cc:27:15: note: within this loop
  for(int i=1;i<=20000004;i++)
              ~^~~~~~~~~~