n = gets.to_i
bef = -1
n.times do |i|
    a = gets.split.map(&:to_i)
    d = a[1].to_i - a[0].to_i
    if d <= 0
        puts -1
        exit
    else
        if bef == -1
            bef = d
        end    
        if d != bef
            puts -1
            exit
        end
        bef = d
    end
end
puts bef