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

N = gets.to_i

m = Hash.new 0

N.times do
	x, y = gets.split.map(&:to_i)
	d = y - x
	if d <= 0
		puts '-1'
		exit
	end
	m[d] += 1
end

if m.keys.size > 1
	puts '-1'
	exit
end

puts m.keys[0]