/in/foo.cc: In function 'int main()':
/in/foo.cc:13:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 1; i <= s1.length(); i++)
                  ~~^~~~~~~~~~~~~~
/in/foo.cc:15:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 1; i <= s2.length(); i++)
                  ~~^~~~~~~~~~~~~~
/in/foo.cc:18:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 1; i <= s1.length(); i++)
                  ~~^~~~~~~~~~~~~~
/in/foo.cc:19:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for (int j = 1; j <= s2.length(); j++)
                   ~~^~~~~~~~~~~~~~