foo.cc: In function 'void pre()':
foo.cc:13:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(int j=0;j<prim.size()&&i*prim[j]<=100000000;j++){
               ~^~~~~~~~~~~~
foo.cc: In function 'bool check(int)':
foo.cc:22:6: warning: unused variable 'j' [-Wunused-variable]
  int j, k = i;
      ^
foo.cc: In function 'int main()':
foo.cc:34:7: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
  scanf("%d", &n);
  ~~~~~^~~~~~~~~~

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