#include <bits/stdc++.h>
using namespace std;
template <typename T>
ostream& operator<<(ostream& os, const vector<T>& v) {
  for (auto i : v) os << i << " ";
  return os;
}
template <typename T>
ostream& operator<<(ostream& os, const set<T>& v) {
  for (auto i : v) os << i << " ";
  return os;
}
const int N = 233;
const int M = 1 << 15;
const int B = 15;
int n;
int a[N], b[N];
using node = pair<vector<int>, int>;
vector<node> State;
int main() {
  scanf("%d", &n);
  for (int i = 1; i <= n; ++i) scanf("%d", &a[i]);
  for (int st = 0; st < M; ++st) {
    for (int i = 1; i <= n; ++i)
      b[i] = __builtin_popcount((a[i] & (M - 1)) ^ st);
    vector<int> v;
    for (int i = 1; i < n; ++i) v.push_back(b[i] - b[i + 1]);
    State.emplace_back(v, st);
  }
  sort(State.begin(), State.end());
  for (int st = 0; st < M; ++st) {
    for (int i = 1; i <= n; ++i) b[i] = __builtin_popcount(a[i] >> 15 ^ st);
    vector<int> v;
    for (int i = 1; i < n; ++i) v.push_back(b[i + 1] - b[i]);
    auto it = lower_bound(State.begin(), State.end(), node(v, 0));
    if (it != State.end() && it->first == v)
      return 0 * printf("%d\n", (st << 15) | it->second);
  }
  printf("-1\n");
  return 0;
}
