/in/foo.c: In function 'main':
/in/foo.c:4:10: error: redeclaration of 'n' with no linkage
  int n,i,n,sum;
          ^
/in/foo.c:4:6: note: previous declaration of 'n' was here
  int n,i,n,sum;
      ^
/in/foo.c:4:12: warning: unused variable 'sum' [-Wunused-variable]
  int n,i,n,sum;
            ^~~
/in/foo.c:4:8: warning: unused variable 'i' [-Wunused-variable]
  int n,i,n,sum;
        ^