#include <bits/stdc++.h>
using namespace std;
int const N = 100;
void solve(map<vector<int>, int>& mp, vector<int> const& a, int m, int off) {
  auto t = a;
  for (int i = 0; i < (1 << m); ++i) {
    for (int j = 0; j < (int)a.size(); ++j)
      t[j] = __builtin_popcount(((a[j] >> off) ^ i) & ((1 << m) - 1));
    int mi = *min_element(t.begin(), t.end());
    for (auto& v : t) v -= mi;
    mp[t] = i << off;
  }
}
void debug_(pair<vector<int>, int> const& a, pair<vector<int>, int> const& b) {
  cerr << ">> " << a.second << ' ' << b.second << '\n';
  cerr << "A : ";
  for (auto const& v : a.first) cerr << v << ' ';
  cerr << '\n';
  for (auto const& v : a.first) cerr << __builtin_popcount(v ^ a.second) << ' ';
  cerr << '\n';
  cerr << "B : ";
  for (auto const& v : b.first) cerr << v << ' ';
  cerr << '\n';
  for (auto const& v : b.first) cerr << __builtin_popcount(v ^ b.second) << ' ';
  cerr << '\n';
}
int main() {
  int n;
  scanf("%d", &n);
  auto a = vector<int>(n, 0);
  for (auto& v : a) scanf("%d", &v);
  auto mp1 = map<vector<int>, int>();
  auto mp2 = map<vector<int>, int>();
  solve(mp1, a, 15, 0);
  solve(mp2, a, 15, 15);
  int ans = -1;
  for (auto const& p : mp1) {
    auto b = p.first;
    auto mx = *max_element(b.begin(), b.end());
    for (auto& v : b) v = mx - v;
    if (mp2.count(b)) {
      ans = p.second | mp2[b];
      break;
    }
  }
  printf("%d\n", ans);
}
