#include <bits/stdc++.h>

using namespace std;

#define F first
#define S second
#define pb push_back
#define mp make_pair
#define eb emplace_back

int rnd() {
    return (rand() << 16) ^ rand();
}

const int N = 200100;
const int M = 63;

pair<int,int> dp[M][N][2];
int cnt[M];

#define FILE "xor-cypher"
int main() {
#ifdef LOCAL
    freopen("input.txt", "r", stdin);
    freopen("output.txt", "w", stdout);
#else
    freopen(FILE".in", "r", stdin);
    freopen(FILE".out", "w", stdout);
#endif
    for (int i = 0; i < M; i++){
        for (int j = 0; j < N; j++){
            for (int q = 0; q < 2; q++){
                dp[i][j][q] = {3, 1e9};
            }
        }
    }
    int n;
    cin >> n;
    long long S;
    cin >> S;
    for (int i = 0; i < n; i++){
        long long x;
        cin >> x;
        for (int j = 0; j < M; j++){
            cnt[j] += ((x & (1ll << j)) != 0);
        }
    }

    int kek[M];
    for (int i = 0; i < M; i++){
        kek[i] = ((S & (1ll << i)) != 0);
    }

    dp[0][0][0] = dp[0][0][1] = {0, 0};
    for (int i = 0; i < M - 1; i++){
        for (int j = 0; j < N; j++){
            for (int q = 0; q < 2; q++){
                if (dp[i][j][q].F == 3) continue;
                int cur;

                cur = j + cnt[i];
                if (cur % 2 == kek[i] && (cur >> 1) < N){
                    dp[i + 1][cur >> 1][0] = min(dp[i + 1][cur >> 1][0], {q, j});
                }

                cur = j + (n - cnt[i]);
                if (cur % 2 == kek[i] && (cur >> 1) < N){
                    dp[i + 1][cur >> 1][1] = min(dp[i + 1][cur >> 1][1], {q, j});
                }
            }
        }
    }

    pair<int,int> y = {3, -1};
    for (int q = 1; q >= 0; q--){
        if (dp[M - 1][0][q].F != 3){
            y = dp[M - 1][0][q];
        }
    }
    if (y.F == 3){
        cout << -1 << endl;
        return 0;
    }
    long long ans = 0;
    for (int i = M - 1; i > 0; i--){
        if (y.F) ans += (1ll << (i - 1));
        y = dp[i][y.S][y.F];
    }
    cout << ans << endl;
    return 0;
}