#include <bits/stdc++.h>
using namespace std;
int n;
int numbers[100][30] = {};
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cin >> n;
  int x;
  for (int i = 0; i < n; i++) {
    cin >> x;
    for (int j = 0; j < 30; j++) {
      numbers[i][j] = x % 2;
      x /= 2;
    }
  }
  map<vector<int>, int> m;
  for (int i = 0; i < (1 << 15); i++) {
    vector<int> b;
    int minx = 15;
    for (int j = 0; j < n; j++) {
      int cnt = 0;
      for (int k = 0; k < 15; k++)
        cnt += (numbers[j][k] ^ (((1 << k) & i) != 0));
      b.push_back(cnt);
      minx = min(minx, cnt);
    }
    for (int j = 0; j < n; j++) b[j] -= minx;
    m[b] = i;
  }
  for (int i = 0; i < (1 << 15); i++) {
    vector<int> b;
    int minx = 15;
    for (int j = 0; j < n; j++) {
      int cnt = 0;
      for (int k = 0; k < 15; k++)
        cnt += ((numbers[j][k + 15]) ^ (((1 << k) & i) != 0));
      b.push_back(cnt);
      minx = min(minx, cnt);
    }
    int maxx = 0;
    for (int j = 0; j < n; j++) {
      b[j] -= minx;
      maxx = max(maxx, b[j]);
    }
    for (int j = 0; j < n; j++) b[j] = maxx - b[j];
    if (m.find(b) != m.end()) {
      cout << m[b] + (1 << 15) * i;
      return 0;
    }
  }
  cout << -1;
}
