foo.cc: In function 'int main()':
foo.cc:7:6: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
 scanf("%d",&n);
 ~~~~~^~~~~~~~~
foo.cc:9:6: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
 scanf("%d",&a[i]);
 ~~~~~^~~~~~~~~~~~
foo.cc:10:6: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
 scanf("%d",&k);
 ~~~~~^~~~~~~~~

自豪的采用HydroJudge(https://github.com/hydro-dev/Hydro)进行评测。