let f (s:string) = s.Split ' ' |> Seq.map int
let uniqCount = Seq.distinct >> Seq.length

let isValid0 n s = n = (s |> f |> uniqCount)
let isValid1 s =
    let cache = s |> (f >> Seq.sort >> Seq.cache)
    1 = (Seq.zip cache (Seq.skip 1 cache)
         |> Seq.map (fun e -> (snd e) - (fst e))
         |> uniqCount)

let isValid n s = (isValid0 n s) && (isValid1 s)

if isValid (stdin.ReadLine () |> int) (stdin.ReadLine ()) then
    "YES"
else
    "NO"
|> printfn "%s"