def lscan; gets.split.map(&:to_i); end

a,b = lscan

if [a,b].min == 1
  p 0
elsif a.gcd(b) == 1
  l = a.lcm(b)+1
  v = [1]*(l)
  v[0] = 0
  (l).times do |i|
    if v[i] == 0
      v[i+a] = 0 if v[i+a]
      v[i+b] = 0 if v[i+b]
    end
  end
  p v.reduce(:+)
else
  p -1
end