foo.cc: In function 'int main()':
foo.cc:10:23: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   10 |         for (int i=0;i<a.size();i++) ans=(ans+a[i]-'0')%9;
      |                      ~^~~~~~~~~
foo.cc:11:23: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   11 |         for (int i=0;i<b.size();i++) ans=(ans+b[i]-'0')%9;
      |                      ~^~~~~~~~~

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