let m = pown 10 9 |> (+) 7 |> int64
let addMod x y = (x + y) % m

let n = stdin.ReadLine() |> int64

let a =
    stdin.ReadLine().Split()
    |> Array.map int64

let rec loop xs =
    let len = Array.length xs
    if len > 1 then
        xs.[1..]
        |> Array.mapi (fun i y -> addMod xs.[i] y)
        |> loop
    elif len = 1 then
        xs.[0]
    else
        -1L

loop a
|> printfn "%d"