from sys import stdin

def erastosthenes(n):
    prime = [0, 1] * ((n + 1) // 2)
    if not n % 2:
        prime += [0]
    prime[1], prime[2] = 0, 1

    i = 3
    while i <= n ** 0.5:
        for j in range(i * i, n + 1, i):
            prime[j] = 0
        i += 2
    return prime

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

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

    n = A + B
    lst = erastosthenes(n // 2)
    prime_list = [i for i in range(0, n // 2 + 1, 2) if lst[i]]
    for i in prime_list:
        if not n % i:
            print(i)
            break
    else:
        print(-1)

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