A,B=map(int,input().split())
x=A+B
import math
xr=math.ceil(math.sqrt(x))

LIST=[]
for i in range(1,xr+1):
    if x%i==0:
        LIST.append(i)
        LIST.append(x//i)

LIST.sort()

for l in LIST:
    if (A+l)%B==0 and (B+l)%A==0 and A!=l and B!=l and A!=B:
        print(l)
        break
else:
    print(-1)