#include <bits/stdc++.h>
using namespace std;
class vector_int_hash {
 public:
  size_t operator()(vector<int> const& vec) const {
    size_t seed = vec.size();
    for (auto& i : vec) {
      seed ^= i + 0x9e3779b9 + (seed << 6) + (seed >> 2);
    }
    return seed;
  }
};
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);
  int n;
  cin >> n;
  vector<int> v;
  for (int i = 0; i < n; ++i) {
    int x;
    cin >> x;
    v.push_back(x);
  }
  unordered_map<vector<int>, int, vector_int_hash> s;
  for (int i = 0; i < (1 << 15); ++i) {
    vector<int> p;
    for (int e : v) {
      p.push_back(__builtin_popcount(e % (1 << 15) ^ i));
    }
    int m = *min_element(p.begin(), p.end());
    for (int& e : p) e -= m;
    if (s.count(p) == 0) {
      s[p] = i;
    }
  }
  for (int i = 0; i < (1 << 15); ++i) {
    vector<int> p;
    for (int e : v) {
      p.push_back(__builtin_popcount(e / (1 << 15) ^ i));
    }
    int m = *min_element(p.begin(), p.end());
    for (int& e : p) e -= m;
    m = *max_element(p.begin(), p.end());
    for (int& e : p) e = m - e;
    if (s.count(p) > 0) {
      cout << i * (1 << 15) + s[p];
      return 0;
    }
  }
  cout << -1;
}
