#include <bits/stdc++.h>
using namespace std;
int n;
int a[155];
const unsigned long long B = 1e9 + 9;
unordered_map<unsigned long long, vector<int>> c;
bool trial(int x) {
  vector<int> b(n);
  for (int i = 0; i < n; i++) {
    b[i] = a[i] ^ x;
  }
  for (int i = 0; i + 1 < n; i++)
    if (__builtin_popcountll(b[i]) != __builtin_popcountll(b[i + 1])) {
      return false;
    }
  return true;
}
int main() {
  cin >> n;
  for (int i = 0; i < n; i++) {
    cin >> a[i];
  }
  for (int mask = 0; mask < (1 << 15); mask++) {
    vector<int> b(n);
    for (int i = 0; i < n; i++) {
      b[i] = (a[i] >> 15) ^ mask;
    }
    unsigned long long h = 0;
    for (int i = 1; i < n; i++) {
      int delta = __builtin_popcountll(b[i]) - __builtin_popcountll(b[0]);
      delta += 15;
      h = h * B + delta;
    }
    c[h].push_back(mask);
  }
  for (int mask = 0; mask < (1 << 15); mask++) {
    vector<int> b(n);
    for (int i = 0; i < n; i++) {
      b[i] = (a[i] & ((1 << 15) - 1)) ^ mask;
    }
    unsigned long long h = 0;
    for (int i = 1; i < n; i++) {
      int delta = __builtin_popcountll(b[0]) - __builtin_popcountll(b[i]);
      delta += 15;
      h = h * B + delta;
    }
    if (!c.count(h)) continue;
    for (auto u : c[h]) {
      if (trial((u << 15) | mask)) {
        printf("%d\n", (u << 15) | mask);
        return 0;
      }
    }
  }
  puts("-1");
  return 0;
}
