/in/foo.c: In function 'main':
/in/foo.c:6:14: error: 'n' undeclared (first use in this function)
  scanf("%d",&n);
              ^
/in/foo.c:6:14: note: each undeclared identifier is reported only once for each function it appears in
/in/foo.c:5:11: warning: unused variable 'a' [-Wunused-variable]
     int i,a;
           ^