/in/foo.cc: In function 'int main()':
/in/foo.cc:11:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i = 0;i < s1.size();i++){
                ~~^~~~~~~~~~~
/in/foo.cc:26:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(int i = 0;i < s1.size() - 1;i++){
                 ~~^~~~~~~~~~~~~~~
/in/foo.cc:35:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(int i = 0;i < s1.size() - 1;i++){
                 ~~^~~~~~~~~~~~~~~