import strutils, sequtils
proc readLine: auto = readLine stdin

var
  a, b = newSeqWith(0, 0)
  even, odd: bool
  maxW, cnt = 0
let
  N = readLine().parseInt

for i in 0 ..< N:
  let t = readLine().split.map parseInt
  a.add t[0]
  b.add t[1]

  case a[^1] mod 2:
    of 0: even = true
    else: odd = true

  if even and odd:
    echo -1
    quit()

  maxW = max(maxW, a[^1] + 4 * b[^1])

for i in 0 ..< N:
  cnt += (maxW - a[i] - 4 * b[i]) div 2

echo cnt