n = int(input())

def f(n):
  total = 0
  width = []
  for i in range(n):
    a, b = map(int, input().split())
    width.append(a + b*4)
  maximum = max(width)

  for i in width:
    if (maximum - i) % 2 == 0:
      total += (maximum - i) / 2
    else:
      return -1

  return int(total)

print(f(n))