foo.c: In function 'main':
foo.c:7:10: warning: unused variable 'f' [-Wunused-variable]
  int i,c,f;
          ^
foo.c:8:2: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
  scanf("%d",&a);
  ^~~~~~~~~~~~~~
foo.c:11:6: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
      scanf("%d",&c);
      ^~~~~~~~~~~~~~

[Hydro](https://hydro.ac)提供评测服务