foo.cc: In function 'int main()':
foo.cc:37:51: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   37 |                                 for (int j = 0; j < a[i].size() - 1; j++)
      |                                                 ~~^~~~~~~~~~~~~~~~~

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