#include <bits/stdc++.h>
using namespace std;
int n;
bool cmp(const vector<int>& l, const vector<int>& r) {
  for (int i = 0; i < n; i++) {
    if (l[i] != r[i]) {
      return l[i] < r[i];
    }
  }
  return false;
}
bool eq(const vector<int>& l, const vector<int>& r) {
  for (int i = 0; i < n; i++) {
    if (l[i] != r[i]) {
      return false;
    }
  }
  return true;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  cin >> n;
  vector<int> a(n);
  for (auto& x : a) {
    cin >> x;
  }
  int mx = (1 << 15) - 1;
  vector<vector<int>> nums(mx + 1, vector<int>(n + 1));
  for (int i = 0; i <= mx; i++) {
    auto& v = nums[i];
    for (int j = 0; j < n; j++) {
      v[j] = __builtin_popcount((a[j] & mx) ^ i);
    }
    v.back() = i;
  }
  sort(nums.begin(), nums.end(), cmp);
  int res = -1;
  vector<int> v(n);
  for (int i = 0; i <= mx; i++) {
    int mn = 32;
    for (int j = 0; j < n; j++) {
      v[j] = 32 - __builtin_popcount((a[j] >> 15) ^ i);
      mn = min(v[j], mn);
    }
    for (int k = 0; k < mn; k++) {
      auto it = lower_bound(nums.begin(), nums.end(), v, cmp);
      if (it != nums.end() && eq(v, *it)) {
        res = (i << 15) ^ it->back();
        i = mx + 1;
        break;
      }
      for (int j = 0; j < n; j++) {
        --v[j];
      }
    }
  }
  cout << res << "\n";
  return 0;
}
