foo.cc: In function 'int main()':
foo.cc:22:7: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
   22 |  scanf("%d", &n);
      |  ~~~~~^~~~~~~~~~
foo.cc:26:8: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
   26 |   scanf("%d", &a[i]);
      |   ~~~~~^~~~~~~~~~~~~
foo.cc:19:8: warning: iteration 1000009 invokes undefined behavior [-Waggressive-loop-optimizations]
   19 |   f[i] = i;
      |   ~~~~~^~~
foo.cc:18:20: note: within this loop
   18 |  for (int i = 1; i <= maxn; i++)
      |                  ~~^~~~~~~

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