n = gets.to_i
m = []
n.times do
  a, b = gets.split.map(&:to_i)
  m.push(a + 4 * b)
end
line = m.max
sum = 0
for i in 0...n do
  diff = line - m[i]
  if diff % 2 == 0
    sum += diff / 2
  else
    sum = -1
    break
  end
end
puts sum