/* -*- coding: utf-8 -*-
 *
 * 3547.cc:  No.3547 Rurumaru Function Problem - yukicoder
 */

#include<cstdio>
#include<algorithm>

using namespace std;

/* constant */

const int BN = 30;

/* typedef */

/* global variables */

/* subroutines */

/* main */

int main() {
  int n, m;
  scanf("%d%d", &n, &m);

  int x = 0;
  for (int i = 0; i < 30 && (n > 0 || m > 0); i++, n >>= 1, m >>= 1) {
    int bn = (n & 1), bm = (m & 1);
    if (! (i & 1)) {
      if (bn == 0 && bm == 1) { puts("-1"); return 0; }
      x |= (bm << i);
    }
    else {
      if (bn == 1 && bm == 0) { puts("-1"); return 0; }
      if (bn == 0) x |= (bm << i);
    }
  }

  printf("%d\n", x);

  return 0;
}