/in/foo.cc: In function 'int main()':
/in/foo.cc:13:7: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   if(i<s1.size())a=s1[i]-'0';
      ~^~~~~~~~~~
/in/foo.cc:14:7: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   if(i<s2.size())b=s2[i]-'0';
      ~^~~~~~~~~~