#include <bits/stdc++.h>
using namespace std;
void solve() {
  int n;
  cin >> n;
  vector<int> arr(n);
  for (int i = 0; i < n; i++) cin >> arr[i];
  map<vector<int>, int> counter, num;
  int mid = 15;
  for (int i = 0; i < (1 << mid); i++) {
    vector<int> temp(n, 0);
    vector<int> count(n, 0);
    for (int j = 0; j < n; j++) {
      count[j] = __builtin_popcount((arr[j] >> mid) ^ (i));
      if (j > 0) temp[j] = count[j] - count[j - 1];
    }
    counter[temp]++;
    if (num.find(temp) == num.end()) num[temp] = (i << 15);
  }
  long long int ans = 0;
  for (int i = 0; i < (1 << mid); i++) {
    vector<int> temp(n, 0);
    vector<int> count(n, 0);
    for (int j = 0; j < n; j++) {
      count[j] = __builtin_popcount((arr[j] & ((1 << mid) - 1)) ^ (i));
      if (j > 0) temp[j] = -(count[j] - count[j - 1]);
    }
    if (counter[temp] > 0) {
      ans = i | num[temp];
      cout << ans << endl;
      return;
    }
  }
  cout << -1 << endl;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int t = 1;
  while (t--) {
    solve();
  }
  return 0;
}
