open System

[<EntryPoint>]
let main argv = 
    let rec readLines n=function
        | lst when n=0 -> lst
        | lst -> 
            let line = Console.ReadLine().Split(' ') 
            (Seq.head line |> int, Seq.last line |> int) :: lst |> readLines (n-1)

    let rec solve n lst=
        match lst, n with
        | (x,y)::tail, None -> solve (Some (y-x)) tail
        | (x,y)::tail, Some n -> if n=y-x then solve (Some n) tail else -1
        | [], Some n when n > 0 -> n
        | _, _ -> -1

    let N = Console.ReadLine () |> int
    [] |> (readLines N) |> solve None |> printfn "%d"
    0