import sys
input = sys.stdin.readline

# from collections import deque

def linput(_t=int):
    return list(map(_t, input().split()))

def gcd(n,m):
    while m: n,m = m, n%m
    return n

def lcm(n,m): return n*m//gcd(n,m)

def main():
    # N = int(input())
    # N,K = linput()
    A,B = linput()
    # vA = linput()
    # vE = [linput() for _ in [0,]*M]

    if B==0:
        print(1)
        return

    if A+1==0:
        print(2)
        return

    res = -1
    print(res)

    # print(("No","Yes")[res%2])


main()