#pragma GCC optimize("O3")
#include<bits/stdc++.h>

#define ll long long
#define pb push_back
#define ld long double
#define f first
#define s second

using namespace std;
const ll inf = (1ll << 62);
int32_t main() {

    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    ll t;
    cin >> t;
    while(t--) {
        ll n, x;
        cin >> n >> x;

        ll ma = n, mi = inf;
        ll mask = 0;
        for(ll i = 0; i < 61; i++) {
            if(!((n >> i) & 1)) {
                if((x >> i) & 1) {
                    mi = -1;
                    break;
                }
            } else {
                ll now = n + (1ll << i) - (n & mask);
                if((x >> i) & 1) {
                    mi = min(mi, now);
                } else {
                    ma = max(ma, now);
                }
            }
            mask += (1ll << i);
        }
        if(ma < mi) cout << ma << '\n';
        else cout << "-1\n";
    }


}