/in/foo.cc: In function 'int main()':
/in/foo.cc:10:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i=0;i<a1.size();i++){
               ~^~~~~~~~~~
/in/foo.cc:18:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i=0;i<b1.size();i++){
               ~^~~~~~~~~~
/in/foo.cc:35:12: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (i=0;i<a1.size();i++){
           ~^~~~~~~~~~
/in/foo.cc:41:12: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (i=0;i<b1.size();i++){
           ~^~~~~~~~~~