foo.c: In function 'hw':
foo.c:5:18: warning: unused variable 'c' [-Wunused-variable]
  int a=n,sum=0,b,c;
                  ^
foo.c: In function 'main':
foo.c:19:10: warning: unused variable 'sum' [-Wunused-variable]
  int n,a,sum=0;
          ^~~
foo.c:19:8: warning: unused variable 'a' [-Wunused-variable]
  int n,a,sum=0;
        ^
foo.c:20:2: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
  scanf("%d",&n);
  ^~~~~~~~~~~~~~

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