foo.cc: In function 'int main()':
foo.cc:12:27: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   12 |         for (int i = 2; i < a.size(); i++)
      |                         ~~^~~~~~~~~~

[Hydro](https://hydro.ac)提供评测服务