n=STDIN.gets.to_i

oddeven=0
max1=0
sum=0

n.times do |i|
	a,b=STDIN.gets.split(" ")
	l=a.to_i+4*b.to_i
	max1=[l,max1].max
	
	if l%2==1 then
		oddeven=1|oddeven
	else
		oddeven=2|oddeven
	end
	sum+=l
end
if oddeven==3 then
	puts -1
else
	puts (max1*n-sum)/2
end