local function getgcd(x, y)
  while 0 < x do
    x, y = y % x, x
  end
  return y
end
local a, b = io.read("*n", "*n")
local gcd = getgcd(a, b)
if 1 < gcd then
  print(-1)
else
  local t = {}
  for i = 1, a * b do t[i] = 1 end
  for i = 0, b do
    for j = 0, a do
      local dst = i * a + j * b
      if 0 < dst and dst <= a * b then
        t[dst] = 0
      end
    end
  end
  local c = 0
  for i = 1, a * b do c = c + t[i] end
  print(c)
end