require 'prime'

a,b = gets.split.map(&:to_i)
d = Prime.prime_division(a+b)

$p = []

def div(c,d,i)
    if i == d.size
        $p << c
    else
        div(c,d,i+1)
        d[i][1].times do |j|
            div(c*(d[i][0]**(j+1)),d,i+1)
        end
    end
end

div(1,d,0)

$p.sort!

$p.each do |i|
    if (a+b)%i == 0 && (b+i)%a == 0 && (i+a)%b == 0 && a != i && b != i
        puts i
        exit
    end
end
puts "-1"