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

fun readLargeInt () =
    valOf (TextIO.scanStream (LargeInt.scan StringCvt.DEC) TextIO.stdIn)


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

        val remType = (#1 (List.hd ab_s)) mod 2

        val findMaxResult =
            List.foldl
                (fn ((a, b), acc) =>
                    case acc of
                        NONE => NONE
                      | SOME min =>
                        if a mod 2 = remType
                        then
                            (
                              if min < a + 4 * b
                              then SOME (a + 4 * b)
                              else SOME min
                            )
                        else NONE)
                (SOME 0)
                ab_s

        val ans = case findMaxResult of
                      NONE => "-1"
                    | SOME max =>
                      LargeInt.toString
                          (List.foldl
                               (fn ((a, b), acc) =>
                                   acc + ((max - (a + 4 * b)) div 2))
                               0
                               ab_s
                          )
    in
        print (ans ^ "\n")
    end