let split (s:string) = s.Split ' '

let subtract = split >> Array.map int >> fun (a:int[]) -> a.[1] - a.[0]
let judge (a:int[]) =
    match a with
    | a when Array.length a = 1 && a.[0] > 0 -> a.[0]
    | _ -> -1

let iter (n:int) =
    seq { for _ in 1..n -> stdin.ReadLine () }
    |> Seq.map subtract
    |> Seq.distinct
    |> Seq.toArray
    |> judge

stdin.ReadLine () |> int |> iter |> printfn "%d"