foo.cc: In function 'int main()':
foo.cc:10:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < a.size(); i++) x = x*10+a[i]-'0';
                  ~~^~~~~~~~~~
foo.cc:11:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < b.size(); i++) y = y*10+b[i]-'0';
                  ~~^~~~~~~~~~

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