import math, sequtils, strutils
let
  n: int = stdin.readline.parseInt
  a: seq[seq[int]] = mapIt(0..<n, stdin.readline.split.map(parseInt))
  x: seq[int] = a.mapIt(it[0] + 4 * it[1])
var ans: int = 0
if x.allIt(it mod 2 == 0) or x.allIt(it mod 2 == 1):
  ans = sum(x.mapIt((x.max - it) div 2))
else:
  ans = -1
echo ans