/in/foo.cc: In function 'int main()':
/in/foo.cc:12:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int j=0;j<a.size();j++)
              ~^~~~~~~~~
/in/foo.cc:14:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int k=0;k<b.size();k++)
              ~^~~~~~~~~