foo.cc: In function 'int main()':
foo.cc:9:11: error: 'pos' was not declared in this scope
  cout << (pos(2, n - 1) * 4 - 2);
           ^~~
foo.cc:9:11: note: suggested alternative: 'pow'
  cout << (pos(2, n - 1) * 4 - 2);
           ^~~
           pow

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