// Learn more about F# at http://fsharp.org

open System

let rec ok (c: List<int>) (seen: Set<int>) =
        if c.IsEmpty then
            true
        else
            let x = c.Head
            if seen.Contains x
            then false
            else ok (c |> List.skipWhile (fun y -> y = x)) (seen.Add x)

[<EntryPoint>]
let main argv =
    let n = stdin.ReadLine() |> int

    let a =
        stdin.ReadLine().Split()
        |> Array.map (fun x -> int (x) - 1)
        |> Array.toList
    
    if (ok a (new Set<int>([]))) then
        printfn "0"
        exit 0

    let mutable freq = Array.create n 0
    for x in a do
        freq.[x] <- freq.[x] + 1
    if freq |> Array.forall (fun v -> v <= 1) then
        printfn "0"
        exit 0
    let x = freq |> Array.findIndex (fun v -> v >= 2)
    let b =
        List.append (a |> List.skipWhile (fun y -> y = x))
            (a |> List.takeWhile (fun y -> y = x))

    printfn "%d" (if (ok b (new Set<int>([]))) then 1 else -1)
    0 // return an integer exit code