/in/foo.cc: In function 'int main()':
/in/foo.cc:12:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i = 1;i <= a.length();i++)
                ~~^~~~~~~~~~~~~
/in/foo.cc:13:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int j = 1;j <= b.length();j++)
                ~~^~~~~~~~~~~~~