/in/foo.cc: In function 'int main()':
/in/foo.cc:24:37: warning: iteration 1008 invokes undefined behavior [-Waggressive-loop-optimizations]
     for(int i=2;i<=n;i++) if(!form[i])
                               ~~~~~~^
/in/foo.cc:24:18: note: within this loop
     for(int i=2;i<=n;i++) if(!form[i])
                 ~^~~