foo.cc: In function 'int main()':
foo.cc:9:10: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
     scanf("%d%d", &n, &k);//输入
     ~~~~~^~~~~~~~~~~~~~~~
foo.cc:13:18: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
             scanf("%d", &a);
             ~~~~~^~~~~~~~~~

[Hydro](https://hydro.ac)提供评测服务