def max(a,b); a > b ? a : b; end
  
N = gets.to_i
A = [0, 0]
max_w = 0
L = N.times.map do
  a,b = gets.split.map(&:to_i)
  A[a % 2] += 1
  (a + 4 * b).tap do |l|
    max_w = max(max_w, l)
  end
end

if A[0] > 0 && A[1] > 0
  puts -1
else
  puts L.inject(0){|s,l| s + (max_w - l) / 2 }
end