# https://yukicoder.me/problems/no/3547

from collections import deque

def main():
    N, M = map(int, input().split())

    n_array = []
    for _ in range(30):
        n_array.append(N % 2)
        N //= 2    
    m_array = []
    for _ in range(30):
        m_array.append(M % 2)
        M //= 2

    answer = 0
    for i in range(30):
        if i % 2 == 0:
            type_tuple = (n_array[i], m_array[i])
            if type_tuple == (0, 0):
                continue
            elif type_tuple == (0, 1):
                print(-1)
                return
            elif type_tuple == (1, 0):
                continue
            elif type_tuple == (1, 1):
                answer += 2 ** i
        else:
            type_tuple = (n_array[i], m_array[i])
            if type_tuple == (0, 0):
                continue
            elif type_tuple == (0, 1):
                answer += 2 ** i
            elif type_tuple == (1, 0):
                print(-1)
                return
            elif type_tuple == (1, 1):
                continue
    print(answer)






if __name__ == "__main__":
    main()