A,B = map(int,input().split())
C = set()
for i in range(1,A+B+1):
    if i*i>A+B:break
    if (A+B)%i==0:
        C.add(i)
        C.add((A+B)//i)
C = sorted(list(C))
flag = -1
for c in C:
    if (A+c)%B==0 and (B+c)%A==0 and c!=A and c!=B:
        flag = c
        break
print(flag)