foo.cc: In function 'int main()':
foo.cc:22:24: warning: iteration 4999998 invokes undefined behavior [-Waggressive-loop-optimizations]
   22 |                 if(!l[i])
      |                     ~~~^
foo.cc:21:22: note: within this loop
   21 |         for(int i=2;i<=5000000;i++)
      |                     ~^~~~~~~~~

[Hydro](https://hydro.ac)提供评测服务