import sequtils
import strutils
import algorithm

proc divisor(n : int) : seq[int] =
  result = newSeq[int](0)
  var n = n
  var i = 1
  while i * i <= n:
    if n mod i == 0:
      result.add(i)
      if i * i != n: result.add(n div i)
    i += 1
  result.sort()

var ab = stdin.readLine.split.map(parseInt)
var (a, b) = (ab[0], ab[1])
var n = a + b
var ans = -1
for c in divisor(n):
  if c != a and c != b:
    if ((c + a) mod b == 0 ) and ((c + b) mod a == 0):
      ans = c
echo ans