import java.math.BigInteger;

public class Main {

  private static void solve() {
    int n = ni();
    long[] a = nal(n);

    int m = 20;
    BigInteger[] b = new BigInteger[1 << m];
    for (int i = 0; i < (1 << m); i++) {
      if (i < n) {
        b[i] = BigInteger.valueOf(a[i]);
      } else {
        b[i] = BigInteger.ZERO;
      }
    }

    for (int j = 0; j < m; j++) {
      for (int i = 0; i < (1 << m); i++) {
        if (b[i].compareTo(BigInteger.ZERO) < 0)
          continue;

        if (((i >> j) & 1) == 0) {

          if (b[i ^ (1 << j)].compareTo(BigInteger.ZERO) < 0) {
            b[i] = BigInteger.valueOf(-1);
            break;
          }
          b[i] = b[i].add(b[i ^ (1 << j)]);
        }
      }
    }

    for (int i = 0; i < (1 << m); i++) {
      if (b[i].compareTo(BigInteger.ZERO) < 0)
        continue;
      b[i] = b[i].multiply(b[i]);
    }

    for (int j = 0; j < m; j++) {
      for (int i = 0; i < (1 << m); i++) {
        if (b[i].compareTo(BigInteger.ZERO) < 0)
          continue;
        if (((i >> j) & 1) == 0) {
          if (b[i ^ (1 << j)].compareTo(BigInteger.ZERO) < 0) {
            b[i] = BigInteger.valueOf(-1);
            break;
          }

          b[i] = b[i].subtract(b[i ^ (1 << j)]);
        }
      }
    }

    BigInteger ret = BigInteger.valueOf(-1);
    for (int i = 0; i < n; i++) {
      if (b[i].compareTo(BigInteger.ZERO) <= 0)
        continue;
      if (ret.compareTo(BigInteger.ZERO) < 0) {
        ret = b[i];
      } else {
        ret = ret.gcd(b[i]);
      }
    }

    System.out.println(ret);
  }

  public static long gcd(long a, long b) {
    while (b > 0) {
      long c = a;
      a = b;
      b = c % b;
    }
    return a;
  }


  public static void main(String[] args) {
    new Thread(null, new Runnable() {
      @Override
      public void run() {
        long start = System.currentTimeMillis();
        String debug = args.length > 0 ? args[0] : null;
        if (debug != null) {
          try {
            is = java.nio.file.Files.newInputStream(java.nio.file.Paths.get(debug));
          } catch (Exception e) {
            throw new RuntimeException(e);
          }
        }
        reader = new java.io.BufferedReader(new java.io.InputStreamReader(is), 32768);
        solve();
        out.flush();
        tr((System.currentTimeMillis() - start) + "ms");
      }
    }, "", 64000000).start();
  }

  private static java.io.InputStream is = System.in;
  private static java.io.PrintWriter out = new java.io.PrintWriter(System.out);
  private static java.util.StringTokenizer tokenizer = null;
  private static java.io.BufferedReader reader;

  public static String next() {
    while (tokenizer == null || !tokenizer.hasMoreTokens()) {
      try {
        tokenizer = new java.util.StringTokenizer(reader.readLine());
      } catch (Exception e) {
        throw new RuntimeException(e);
      }
    }
    return tokenizer.nextToken();
  }

  private static double nd() {
    return Double.parseDouble(next());
  }

  private static long nl() {
    return Long.parseLong(next());
  }

  private static int[] na(int n) {
    int[] a = new int[n];
    for (int i = 0; i < n; i++)
      a[i] = ni();
    return a;
  }

  private static char[] ns() {
    return next().toCharArray();
  }

  private static long[] nal(int n) {
    long[] a = new long[n];
    for (int i = 0; i < n; i++)
      a[i] = nl();
    return a;
  }

  private static int[][] ntable(int n, int m) {
    int[][] table = new int[n][m];
    for (int i = 0; i < n; i++) {
      for (int j = 0; j < m; j++) {
        table[i][j] = ni();
      }
    }
    return table;
  }

  private static int[][] nlist(int n, int m) {
    int[][] table = new int[m][n];
    for (int i = 0; i < n; i++) {
      for (int j = 0; j < m; j++) {
        table[j][i] = ni();
      }
    }
    return table;
  }

  private static int ni() {
    return Integer.parseInt(next());
  }

  private static void tr(Object... o) {
    if (is != System.in)
      System.out.println(java.util.Arrays.deepToString(o));
  }
}