import sys
input=lambda: sys.stdin.readline().rstrip()
a,b=map(int,input().split())
def make_divisors(n):
  divisors=[]
  for i in range(1,int(n**0.5)+1):
    if n%i==0:
      divisors.append(i)
      if i!=n//i:
        divisors.append(n//i)

  divisors.sort()
  return divisors
D=make_divisors(a+b)
for c in D:
  if (b+c)%a==0 and (c+a)%b==0:
    print(c)
    break
else:
  print(-1)