/in/foo.cc: In function 'int main()':
/in/foo.cc:9:12: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  if(x>=0&&x<a.size())cout<<b<<" is substring of "<<a<<endl;
           ~^~~~~~~~~
/in/foo.cc:10:17: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  else if(y>=0&&y<b.size())cout<<a<<" is substring of "<<b<<endl;
                ~^~~~~~~~~