#include <bits/stdc++.h>
using namespace std;
template <typename T1, typename T2>
inline void chkmin(T1 &x, const T2 &y) {
  if (x > y) x = y;
}
template <typename T1, typename T2>
inline void chkmax(T1 &x, const T2 &y) {
  if (x < y) x = y;
}
long long n;
vector<long long> a;
void read() {
  cin >> n;
  a.resize(n);
  for (auto &i : a) cin >> i;
  sort(a.begin(), a.end());
  a.resize(unique(a.begin(), a.end()) - a.begin());
}
long long ans = -1;
vector<long long> make(long long x) {
  vector<long long> ans(n, 0);
  for (long long i = 0; i < n; i++) {
    ans[i] = __builtin_popcount(((a[i] & ((1 << 15) - 1))) ^ x);
  }
  return ans;
}
vector<long long> make2(long long x) {
  vector<long long> ans(n, 0);
  for (long long i = 0; i < n; i++) {
    ans[i] = __builtin_popcount((a[i] >> 15) ^ x);
  }
  return ans;
}
map<vector<long long>, long long> masks;
pair<bool, long long> check(long long len, vector<long long> x) {
  for (auto &i : x) {
    if (i > len) return {false, 0};
    i = len - i;
  }
  if (masks.find(x) != masks.end()) {
    return {true, masks[x]};
  }
  return {false, 0};
}
void run() {
  for (long long mask = 0; mask < (1 << 15); mask++) {
    masks[make(mask)] = mask;
  }
  for (long long len = 0; len <= 30; len++) {
    for (long long mask = 0; mask < (1 << 15); mask++) {
      auto x = make2(mask);
      auto fans = check(len, x);
      if (fans.first) {
        ans = (mask << 15) | fans.second;
        return;
      }
    }
  }
}
void write() { cout << ans << endl; }
signed main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  read();
  run();
  write();
  return 0;
}
