# A+B=n*C
# B+C=m*A
# C+A=l*B
# 2 * (A+B+C) = n*C + m*A + l*B
def divisor(n):
    n2 = int((n**0.5)+1) + 1
    ret = set()
    for i in range(1,n2):
        if n%i==0:
            ret.add(i)
            ret.add(n//i)
    ret = list(ret)
    ret.sort()
    return ret

def solve():
    A,B=map(int,input().split())
    Cs=divisor(A+B)
    #print(Cs)
    for c in Cs:
        if (B+c)%A==0 and (c+A)%B==0 and A!=c and B!=c:
            return c
    return -1

print(solve())