foo.cc: In function 'int main()':
foo.cc:34:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<v.size();i=i+4)
              ~^~~~~~~~~
foo.cc:24:13: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
  int n;scanf("%d\n",&n);
        ~~~~~^~~~~~~~~~~

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