/in/foo.cc: In function 'int main()':
/in/foo.cc:15:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<a.size();i++)
              ~^~~~~~~~~
/in/foo.cc:19:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<b.size();i++)
              ~^~~~~~~~~
/in/foo.cc:24:27: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0, s = 0; i < b.size(); i++, s++, s %= a.size()) {
                         ~~^~~~~~~~~~