#include <bits/stdc++.h>
using namespace std;
std::ostream& operator<<(std::ostream& os, pair<int, int> p) {
  os << p.first << ", " << p.second;
  return os;
}
int main() {
  long long int n;
  cin >> n;
  vector<long long int> dizi(n);
  for (int i = 0; i < n; i++) cin >> dizi[i];
  map<vector<long long int>, int> vls;
  long long int b0masked = dizi[0] & ((1ll << 15) - 1);
  long long int msk = ((1ll << 15) - 1);
  for (long long int i = 0; i < (1 << 15); i++) {
    vector<long long int> hr;
    long long int ri = i;
    for (long long int j = 1; j < n; j++) {
      hr.push_back(__builtin_popcount(b0masked ^ ri) -
                   __builtin_popcount((dizi[j] & msk) ^ ri));
    }
    vls[hr] = ri;
  }
  b0masked = dizi[0] >> 15;
  for (long long int i = 0; i < (1 << 15); i++) {
    long long int ri = i;
    vector<long long int> hr;
    for (long long int j = 1; j < n; j++) {
      hr.push_back(-(__builtin_popcount(b0masked ^ ri) -
                     __builtin_popcount((dizi[j] >> 15) ^ ri)));
    }
    if ((vls.find(hr) != vls.end())) {
      cout << (vls[hr] | (ri << 15)) << "\n";
      return 0;
    }
  }
  cout << -1 << "\n";
  return 0;
}
