foo.cc: In function 'int len(int)':
foo.cc:17:1: warning: control reaches end of non-void function [-Wreturn-type]
   17 | }
      | ^
foo.cc: In function 'int main()':
foo.cc:21:10: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
   21 |     scanf("%d",&n);
      |     ~~~~~^~~~~~~~~

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