n = gets.to_i
a = []
n.times do
  a << gets.split.map(&:to_i)
end
a.map!{|i,j| i + j * 4}
if a.all?{|i| i % 2 == 0} || a.all?{|i| i % 2 == 1}
  m = a.max
  p a.inject(0){|sum,i| sum += m-i} / 2
else
  p -1
end