A, B = gets.split.map(&:to_i)
ans = (1 .. A + B).inject(Float::INFINITY) do |r, c|
  break -1 if r < c
  m, d = (A + B).divmod(c)
  next m if d > 0

  break c if (A + c) % B == 0 && (B + c) % A == 0
  break m if (A + m) % B == 0 && (B + m) % A == 0
  
  next m
end
puts ans