foo.c: In function 'main':
foo.c:4:15: error: 'sum' undeclared (first use in this function)
  int a=1, b=1;sum;
               ^~~
foo.c:4:15: note: each undeclared identifier is reported only once for each function it appears in
foo.c:5:8: warning: unused variable 'i' [-Wunused-variable]
  int n,i;
        ^
foo.c:5:6: warning: unused variable 'n' [-Wunused-variable]
  int n,i;
      ^
foo.c:4:11: warning: unused variable 'b' [-Wunused-variable]
  int a=1, b=1;sum;
           ^
foo.c:7:2: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
  scanf("%d", &a);
  ^~~~~~~~~~~~~~~

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