/in/foo.cc: In function 'int main()':
/in/foo.cc:8:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<s1.size();i++){
              ~^~~~~~~~~~
/in/foo.cc:11:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<s2.size();i++){
              ~^~~~~~~~~~
/in/foo.cc:15:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=1;i<=s1.size();i++){
              ~^~~~~~~~~~~
/in/foo.cc:17:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(int j=1;j<=s2.size();j++){
               ~^~~~~~~~~~~