open System open System.Collections.Generic [] module Cin = let read f = stdin.ReadLine() |> f let reada f = stdin.ReadLine().Split() |> Array.map f let readChars() = read string |> Seq.toArray let readInts() = readChars() |> Array.map (fun x -> Convert.ToInt32(x.ToString())) [] module Cout = let writer = new IO.StreamWriter(new IO.BufferedStream(Console.OpenStandardOutput())) let print (s: string) = writer.Write s let println (s: string) = writer.WriteLine s let inline puts (s: ^a) = string s |> println let solve() = let n = read int let xs = reada int64 |> Array.sort let ok1 = Array.tail xs |> Array.forall (fun x -> x <> xs.[0]) let mutable ok2 = true for i in 0 .. n - 3 do let d1 = xs.[i] - xs.[i + 1] |> abs let d2 = xs.[i + 1] - xs.[i + 2] |> abs if d1 <> d2 then ok2 <- false if ok1 && ok2 then "YES" else "NO" |> puts () [] let main _ = try solve() with e -> printfn "%s" (e.ToString()) writer.Close() 0 // return an integer exit code