gets
res = catch(:impossible) do
  ary = []
  max_sp = 0
  while line = gets do
    a, b = line.split.map(&:to_i)
    sp = a + 4 * b
    ary << sp
    max_sp = sp if sp > max_sp
  end

  count = 0
  ary.each do |sp|
    q, r = (max_sp - sp).divmod(2)
    throw :impossible, -1 unless r == 0
    count += q
  end
  count
end

puts res