from sys import stdin

def primeFactors(n):
    factors = []
    apnd = factors.append
    while n >= 4:
        if n % 2:
            break
        else:
            n //= 2
            apnd(2)
    i = 3
    while n >= i * i:
        if n % i:
            i += 2
        else:
            n //= i
            apnd(i)
    apnd(n)
    return factors

def main():
    A, B = map(int, input().split())

    if A == B:
        print(-1)
        return

    p = primeFactors(A + B)
    if len(p) == 1:
        print(-1)
    else:
        for c in p:
            if not (c + A) % B and not (c + B) % A:
                print(c)
                break
        else:
            print(-1)

input = lambda: stdin.readline().rstrip()
main()