foo.cc: In function 'int main()':
foo.cc:8:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
 for (int i=1; i<=s1.length(); i++)
               ~^~~~~~~~~~~~~
foo.cc:9:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
 for (int j=1; j<=s2.length(); j++)
               ~^~~~~~~~~~~~~

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