foo.cc: In function 'int main()':
foo.cc:18:8: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
   scanf("%d",&n);
   ~~~~~^~~~~~~~~
foo.cc:19:35: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
   for (int i = 1;i <= n;i++) scanf("%d",&a[i]);
                              ~~~~~^~~~~~~~~~~~

[Hydro](https://hydro.ac)提供评测服务