#! ruby
# yukicoder My Practice
# author: Leonardone @ NEETSDKASU

N = gets.to_i

ab = []
evens = 0

N.times do
	a, b = gets.split.map(&:to_i)
	sum = a + b * 4
	ab << sum
	evens += 1 if sum.even?
end

if evens > 0 && evens < N
	puts '-1'
	exit
end

abmax = ab.max

count = 0;

ab.each do |x|
	count += [abmax - x, 0].max.div(2)
end

puts count