#include <bits/stdc++.h>
using namespace std;
const int full = 30;
const int half = 15;
void normalizare(vector<int>& v) {
  int n = v.size();
  for (int i = n - 1; i >= 0; i--) v[i] -= v[0];
}
void solve() {
  int n;
  cin >> n;
  vector<int> v(n);
  for (auto& i : v) cin >> i;
  map<vector<int>, int> este;
  for (int mask = 0; mask < (1 << half); mask++) {
    vector<int> cnt(n, 0);
    for (int i = 0; i < n; i++)
      cnt[i] = __builtin_popcount((v[i] & ((1 << half) - 1)) ^ mask);
    normalizare(cnt);
    if (este.find(cnt) == este.end()) este[cnt] = mask;
  }
  for (int mask = 0; mask < (1 << half); mask++) {
    vector<int> cnt(n, 0);
    for (int i = 0; i < n; i++)
      cnt[i] = __builtin_popcount((v[i] >> half) ^ mask);
    normalizare(cnt);
    for (int i = 0; i < n; i++) cnt[i] = -cnt[i];
    if (este.find(cnt) != este.end()) {
      int res = (mask << half) + este[cnt];
      cout << res << '\n';
      return;
    }
  }
  cout << -1 << '\n';
}
int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  solve();
}
