#include <bits/stdc++.h>
using namespace std;
int a[111], n;
map<vector<int>, int> v2i;
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0), cout.tie(0);
  cin >> n;
  for (int i = 0; i < n; i++) cin >> a[i];
  int tot = 1 << 18;
  for (int bt = 0; bt < tot; bt++) {
    vector<int> tv;
    tv.resize(n);
    for (int i = 0; i < n; i++)
      tv[i] = __builtin_popcount(bt ^ (a[i] & (tot - 1)));
    for (int i = n - 1; i >= 0; i--) tv[i] -= tv[0];
    if (v2i.count(tv)) continue;
    v2i[tv] = bt;
  }
  for (int bt = 0; bt < (1 << 12); bt++) {
    vector<int> tv;
    tv.resize(n);
    for (int i = 0; i < n; i++) tv[i] = __builtin_popcount(bt ^ (a[i] >> 18));
    for (int i = n - 1; i >= 0; i--) tv[i] -= tv[0];
    for (int i = 0; i < n; i++) tv[i] = -tv[i];
    if (v2i.count(tv)) {
      if (bt == 0 && v2i[tv] == 0) continue;
      cout << ((bt << 18) | v2i[tv]) << "\n";
      return 0;
    }
  }
  cout << "-1"
       << "\n";
  return 0;
}
