foo.cc: In function 'int cal(const std::vector<int>&, int)':
foo.cc:25:21: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   25 |   for (int j(*i); j != min_stamps.size(); ++j) {
      |                   ~~^~~~~~~~~~~~~~~~~~~~
foo.cc:30:11: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   30 |  for (; i != min_stamps.size(); ++i) {
      |         ~~^~~~~~~~~~~~~~~~~~~~

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