let () =
  let n = Scanf.scanf "%d " (fun i -> i) in
  let x = Array.init n (fun _ -> Scanf.scanf "%d " (fun i -> i)) in
  Array.fast_sort (-) x;
  let rec doit i =
    if i = n then true
    else if x.(i-1) = x.(i) then false
    else doit (i + 1) in
  if not (doit 1) then print_endline "NO"
  else
    let d = x.(1) - x.(0) in
    let rec doit i =
      if i = n then true
      else if x.(i) - x.(i-1) <> d then false
      else doit (i + 1) in
    print_endline (if doit 2 then "YES" else "NO")