fun readInt () =
    valOf (TextIO.scanStream (Int.scan StringCvt.DEC) TextIO.stdIn)

fun intString n =
    if 0 <= n
    then Int.toString n
    else "-" ^ Int.toString (abs n)

fun findAns xy_s =
    let
        val diff = #2 (List.hd xy_s) - #1 (List.hd xy_s)
    in
        if List.all
               (fn (x, y) =>
                   y - x = diff)
               xy_s
           andalso
           0 < diff
        then diff
        else ~1
    end


val () =
    let
        val n = readInt ()
        val xy_s = List.tabulate (n, fn _ => (readInt (), readInt ()))

        val ans = findAns xy_s
        
    in    
        print (intString ans ^ "\n")
    end