let cMajor = [| 0; 2; 4; 5; 7; 9; 11 |]

let getScale key =
    cMajor
    |> Array.map ((+) key >> fun n -> n % 12)

let scales =
    [ 0 .. 11 ]
    |> List.map getScale

let _ = stdin.ReadLine()

let ts =
    stdin.ReadLine().Split()
    |> Array.map int

let fn1 scale =
    ts
    |> Array.forall (fun t -> Array.contains t scale)

scales
|> List.filter fn1
|> fun arr ->
    if arr.Length = 1 then arr.[0].[0]
    else -1
|> printfn "%d"