import strutils, sequtils, algorithm, math

proc solve(): int =
  var
    n = readLine(stdin).parseInt()
    aSeq = newSeq[int](0)
    bSeq = newSeq[int](0)
    maxLen = 0
    result = 0
    ab: seq[int]
    diff: int

  for i in countup(0, n - 1):
    ab = readLine(stdin).split(' ').map(parseInt)
    aSeq.add(ab[0])
    bSeq.add(ab[1])
    maxLen = max(maxLen, ab[0] + 4 * ab[1])

  for i in countup(0, n - 1):
    diff = maxLen - (aSeq[i] + 4 * bSeq[i])
    if diff mod 2 == 0:
      result += diff div 2
    else:
      return -1

  return result

echo(solve())