foo.cc: In function 'int main()':
foo.cc:36:20: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<std::vector<int> >::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   36 |  for (int i = 0; i < M.size(); i++) {
      |                  ~~^~~~~~~~~~
foo.cc:37:21: 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 < M[i].size(); j++) {
      |                   ~~^~~~~~~~~~~~~
foo.cc:41:9: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<std::vector<int> >::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   41 |   if (i != M.size() - 1) {
      |       ~~^~~~~~~~~~~~~~~

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