#include <bits/stdc++.h>
#pragma GCC optimize("O3")
using namespace std;
int n;
const int maxN = 105;
int a[maxN];
int bits[(1 << 15) + 10];
map<vector<int>, int> mp;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);
  bits[0] = 0;
  for (int i = 1; i < (1 << 15); i++) bits[i] = bits[i / 2] + (i & 1);
  cin >> n;
  for (int i = 0; i < n; i++) {
    cin >> a[i];
  }
  for (int mask = 0; mask < (1 << 15); mask++) {
    vector<int> p1(n), p2(n);
    for (int p = 0; p < n; p++) {
      int val1 = a[p] & ((1 << 15) - 1);
      p1[p] = bits[val1 ^ mask];
    }
    int val = p1[0];
    for (int& u : p1) u -= val;
    mp[p1] = mask;
  }
  for (int mask = 0; mask < (1 << 15); mask++) {
    vector<int> p2(n);
    for (int p = 0; p < n; p++) {
      int val1 = a[p] >> 15;
      p2[p] = bits[val1 ^ mask];
    }
    int val = p2[0];
    for (int& u : p2) {
      u -= val;
      u = -u;
    }
    if (mp.count(p2)) {
      int x = mp[p2] + (1 << 15) * mask;
      vector<int> shit;
      for (int p = 0; p < n; p++) {
        int c = a[p] ^ x;
        shit.emplace_back(bits[c >> 15] + bits[c & ((1 << 15) - 1)]);
      }
      for (int u : shit) assert(u == shit[0]);
      cout << x << '\n';
      return 0;
    }
  }
  cout << -1;
  return 0;
}
