n = gets.chomp.to_i

answer = -1, answer_prev = -1
n.times {
    x, y = gets.chomp.split.map &:to_i
    answer = y - x
    if answer_prev == -1
        answer_prev = answer
    end
    if answer <= 0 or not answer_prev == answer
        p -1
        exit
    end
    answer_prev = answer
}
p answer