foo.cc: In function 'int main()':
foo.cc:30:7: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
  scanf("%d", &n);
  ~~~~~^~~~~~~~~~
foo.cc:31:59: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
  for(register int i = 1; i <= n; i++) scanf("%d", &a[i].v), a[i].id = i;
                                       ~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~
foo.cc:32:59: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
  for(register int i = 1; i <= n; i++) scanf("%d", &b[i].v), b[i].id = i;
                                       ~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~

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