#include <bits/stdc++.h>
using namespace std;
namespace TRIE {
struct node {
  node *nxt[31];
  int ang;
};
node *newNode() {
  node *ret = (node *)malloc(sizeof(node));
  for (int i = 0; i < 31; i++) {
    ret->nxt[i] = NULL;
  }
  ret->ang = -1;
  return ret;
}
void add(node *cur, vector<int> &pat, int ang) {
  for (int i = 0; i < pat.size(); i++) {
    if (cur->nxt[pat[i] + 15] == NULL) {
      cur->nxt[pat[i] + 15] = newNode();
    }
    cur = cur->nxt[pat[i] + 15];
  }
  cur->ang = ang;
}
int chk(node *cur, vector<int> &pat) {
  for (int i = 0; i < pat.size(); i++) {
    if (cur->nxt[pat[i] + 15] == NULL) return -1;
    cur = cur->nxt[pat[i] + 15];
  }
  return cur->ang;
}
}  // namespace TRIE
int ar[105];
int main() {
  int n;
  scanf("%d", &n);
  TRIE::node *root = TRIE::newNode();
  for (int i = 0; i < n; i++) {
    scanf("%d", &ar[i]);
  }
  for (int i = (1 << 15) - 1; i >= 0; i--) {
    vector<int> eval;
    for (int j = 0, prv; j < n; j++) {
      int chk = ar[j] ^ i;
      int cnt = 0;
      for (int k = 0; k < 15; k++) {
        if (chk & (1 << k)) cnt++;
      }
      if (j) eval.push_back(cnt - prv);
      prv = cnt;
    }
    TRIE::add(root, eval, i);
  }
  int ans = -1;
  for (int i = (1 << 15) - 1; i >= 0 && ans == -1; i--) {
    vector<int> eval;
    for (int j = 0, prv; j < n; j++) {
      int chk = ar[j] ^ (i << 15);
      int cnt = 0;
      for (int k = 0; k < 15; k++) {
        if (chk & (1 << (k + 15))) cnt++;
      }
      if (j) eval.push_back(prv - cnt);
      prv = cnt;
    }
    int chk = TRIE::chk(root, eval);
    if (chk >= 0) ans = (i << 15) | chk;
  }
  printf("%d\n", ans);
  return 0;
}
