import strutils, sequtils, math
proc readLine: auto = readLine stdin

let
  N = readLine().parseInt
var
  a, b = newSeqWith(N, 0)
  evenOdd = [1, 1]
  maxW, cnt = 0

proc width(i: int): int = a[i] + 4 * b[i]

for i in 0 ..< N:
  (a[i], b[i]) = readLine().split.map parseInt

  evenOdd[a[i] mod 2] *= 0
  if evenOdd.sum == 0:
    echo -1
    quit()

  maxW = max(maxW, i.width)

for i in 0 ..< N:
  cnt += (maxW - i.width) div 2

echo cnt