#約数全部
def Divisors(N):
    N=abs(N)
    L,U=[],[]
    k=1
    while k*k <=N:
        if N%k== 0:
            L.append(k)
            if k!=N//k:
                U.append(N//k)
        k+=1
    return L+U[::-1]
#================================================
A,B=map(int,input().split())
D=Divisors(A+B)

C=-1
for c in D[::-1]:
    if (A+c)%B==(B+c)%A==0 and A!=c and B!=c:
        C=c
print(C)