#include <bits/stdc++.h>
using namespace std;
const long long mod = 1e9 + 7;
template <class T>
inline T read() {
  T x = 0, f = 1;
  char s = getchar();
  while (s < '0' || s > '9') {
    if (s == '-') f = -1;
    s = getchar();
  };
  while (s >= '0' && s <= '9') {
    x = (x << 3) + (x << 1) + s - 48;
    s = getchar();
  };
  return x * f;
}
template <typename T>
inline void wri(T x) {
  if (x < 0) {
    x = -x;
    putchar('-');
  };
  if (x / 10) wri(x / 10);
  putchar(x % 10 + 48);
}
template <typename T>
inline void print(T x, char s) {
  wri(x);
  putchar(s);
}
template <typename T>
T gcd(T a, T b) {
  return b == 0 ? a : gcd(b, a % b);
}
const int maxn = 3e6 + 5;
int a[maxn];
map<vector<int>, int> M;
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0), cout.tie(0);
  int n;
  cin >> n;
  for (register int i = (1); i < (n + 1); i++) cin >> a[i];
  int b = 15, s = (1 << b) - 1;
  for (register int i = (0); i < (s + 1); i++) {
    vector<int> v;
    for (register int j = (2); j < (n + 1); j++)
      v.push_back(__builtin_popcount((a[j] & s) ^ i) -
                  __builtin_popcount((a[1] & s) ^ i));
    M[v] = i;
  }
  for (register int i = (0); i < (s + 1); i++) {
    vector<int> v;
    for (register int j = (2); j < (n + 1); j++)
      v.push_back(__builtin_popcount((a[1] >> b) ^ i) -
                  __builtin_popcount((a[j] >> b) ^ i));
    if (M.find(v) != M.end()) {
      cout << M[v] + (i << b) << endl;
      return 0;
    }
  }
  cout << -1 << endl;
  return 0;
}
