#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ii = pair<int, int>;
const int N = 103;
char pcount[100005];
char pop_count(int x) {
  if (pcount[x] != 40) {
    return pcount[x];
  }
  return pcount[x] = __builtin_popcount(x);
}
int main() {
  for (int i = 0; i < 100005; i++) pcount[i] = 40;
  map<vector<char>, int> ma;
  int n;
  scanf("%d", &n);
  int v[N];
  for (int i = 0; i < n; i++) {
    scanf("%d", v + i);
  }
  int mid = 15;
  for (int mask = 0; mask < 1 << mid; mask++) {
    vector<char> q;
    for (int i = 0; i < n; i++) {
      q.push_back(pop_count((v[i] & ((1 << mid) - 1)) ^ mask));
    }
    ma[q] = mask;
  }
  for (int i = 0; i < n; i++) {
    v[i] >>= mid;
  }
  vector<char> q(n);
  for (int mask = 0; mask < 1 << mid; mask++) {
    for (int j = 0; j <= 2 * mid; j++) {
      for (int i = 0; i < n; i++) {
        q[i] = j - pop_count(v[i] ^ mask);
      }
      if (ma.count(q)) {
        printf("%d\n", ma[q] + (mask << 15));
        return 0;
      }
    }
  }
  printf("-1\n");
  return 0;
}
