#include <bits/stdc++.h>
using namespace std;
const int MD = (1 << 15);
map<vector<int>, int> A[2];
vector<int> N[2];
void llena(int t, int n) {
  for (int mask = 0; mask < MD; mask++) {
    vector<int> aux(n, 0);
    for (int i = 0; i < n; i++) {
      N[t][i] = N[t][i] ^ mask;
      aux[i] = __builtin_popcount(N[t][i]);
      N[t][i] = N[t][i] ^ mask;
    }
    A[t][aux] = mask;
  }
  return;
}
vector<int> opuesto(vector<int> &v, int lvl, int n) {
  vector<int> ans(n, -1);
  for (auto lv : v)
    if (lvl < lv) return ans;
  int con = 0;
  for (auto lv : v) {
    ans[con] = lvl - lv;
    con++;
  }
  return ans;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  cout.precision(10);
  cout << fixed;
  int n, x;
  cin >> n;
  for (int i = 0; i < n; i++) {
    cin >> x;
    N[0].push_back(x % MD);
    N[1].push_back(x / MD);
  }
  llena(0, n);
  llena(1, n);
  for (int i = 0; i <= 30; i++) {
    for (map<vector<int>, int>::iterator par = A[0].begin(); par != A[0].end();
         par++) {
      vector<int> auxi = par->first;
      vector<int> aux = opuesto(auxi, i, n);
      if (aux[0] == -1) continue;
      if (A[1].find(aux) != A[1].end()) {
        cout << (A[1][aux] << 15) + par->second << "\n";
        return 0;
      }
    }
  }
  cout << -1 << "\n";
  return 0;
}
