foo.cc: In function 'int main()':
foo.cc:9:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<s1.size();i++)
              ~^~~~~~~~~~
foo.cc:12:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int j=0;j<s2.size();j++)
              ~^~~~~~~~~~

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