#!/usr/bin/ruby
N=gets.to_i
a=N.times.map{
	a,b=gets.split.map(&:to_i)
	a+4*b
}
if a.all?{|e|e%2==a[0]%2}
	ma=a.max
	p a.reduce(0){|s,e|s+(ma-e)/2}
else
	p -1
end