foo.cc: In function 'int main()':
foo.cc:15:27: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   15 |         for (int j = 1; j < a.size() && a[j] * i < N; j++) {
      |                         ~~^~~~~~~~~~
foo.cc:21:23: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   21 |     for (int i = 1; i <= a.size() && 2 * a[i] <= n; i++) {
      |                     ~~^~~~~~~~~~~

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