/in/foo.cc: In function 'int main()':
/in/foo.cc:12:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(int i = 1;i <= a.length();i++)
                   ~~^~~~~~~~~~~~~
/in/foo.cc:13:25: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         for(int j = 1;j <= b.length();j++)
                       ~~^~~~~~~~~~~~~