/in/foo.cc: In function 'int main()':
/in/foo.cc:10:29: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (long long i = 0; i < s.size(); i += 2)
                           ~~^~~~~~~~~~
/in/foo.cc:23:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         else if (i < s.size() - 2)
                  ~~^~~~~~~~~~~~~~