A,B=map(int, input().split())
ans=[]
C1,C2=abs(A-B),A+B
def f(C):
  a=(A+B)%C
  b=(B+C)%A
  c=(C+A)%B
  if a==0 and b==0 and c==0:
    return True
  else:
    return False
if f(C1):
  ans.append(C1)
if f(C2):
  ans.append(C2)
if len(ans)==0:
  print(-1)
else:
  print(min(ans))