/in/foo.cc: In function 'int main()':
/in/foo.cc:13:13: warning: iteration 3000008 invokes undefined behavior [-Waggressive-loop-optimizations]
         f[i]=sum[i/2]+1,sum[i]=sum[i-1]+f[i];
         ~~~~^~~~~~~~~~~
/in/foo.cc:12:18: note: within this loop
     for(int i=2;i<=maxn;i++)
                 ~^~~~~~