foo.c: In function 'main':
foo.c:5:15: error: 'r' undeclared (first use in this function)
     int n,i,a[r]={0,1}
               ^
foo.c:5:15: note: each undeclared identifier is reported only once for each function it appears in
foo.c:6:2: error: expected ',' or ';' before 'scanf'
  scanf("%d",&n);
  ^~~~~
foo.c:5:13: warning: unused variable 'a' [-Wunused-variable]
     int n,i,a[r]={0,1}
             ^

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