n = gets.to_i
code = []
n.times{
	a,b = gets.chomp.split(" ").map(&:to_i)
	code.push(a + b * 4)
}
MAX = code.max
count = 0
code.each{|value|
	gap = MAX - value
	if gap % 2 == 1
		count = -1
		break
	else
		count += gap / 2
	end
}
puts count