/in/foo.cc: In function 'void initial()':
/in/foo.cc:20:22: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         for(int j=1;j<=cnt&&prime[j]*i<=N;j++)
                     ~^~~~~
/in/foo.cc:27:50: warning: iteration 5999999 invokes undefined behavior [-Waggressive-loop-optimizations]
     for(int i=1;i<=N;i++)sum[i]=sum[i-1]+mobius[i];
                                          ~~~~~~~~^
/in/foo.cc:27:18: note: within this loop
     for(int i=1;i<=N;i++)sum[i]=sum[i-1]+mobius[i];
                 ~^~~
/in/foo.cc:19:19: warning: iteration 5999998 invokes undefined behavior [-Waggressive-loop-optimizations]
         if(!book[i]){prime[++cnt]=i;mobius[i]=-1;}
             ~~~~~~^
/in/foo.cc:17:18: note: within this loop
     for(int i=2;i<=N;i++)
                 ~^~~