foo.cc: In function 'int main()':
foo.cc:18:6: warning: unused variable 'a' [-Wunused-variable]
   18 |  int a[maxn];
      |      ^
foo.cc:20:7: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
   20 |  scanf("%d",&k);
      |  ~~~~~^~~~~~~~~

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