let N = stdin.ReadLine() |> int
let r() = let arr = stdin.ReadLine().Split(' ') |> Array.map int in arr.[1] - arr.[0]
let arr = [ for x in 1..N -> r() ]
let a = arr.[0]
if List.forall ((=)a) arr && arr.[0] > 0 then arr.[0]
else -1
|> printfn "%d"