/in/foo.cc: In function 'int main()':
/in/foo.cc:19:27: warning: iteration 9999998 invokes undefined behavior [-Waggressive-loop-optimizations]
  for(int i=2;i<=n;i++)a[i]=1;
                       ~~~~^~
/in/foo.cc:19:15: note: within this loop
  for(int i=2;i<=n;i++)a[i]=1;
              ~^~~