foo.cc: In function 'int main()':
foo.cc:51:7: error: 'cout' is not a member of 'std'
  std::cout << x;
       ^~~~
foo.cc:51:7: note: suggested alternative: 'count'
  std::cout << x;
       ^~~~
       count

自豪的采用HydroJudge(https://github.com/hydro-dev/Hydro)进行评测。