public class Main {

  private static void solve() {
    int n = ni();
    long[] a = nal(n);
    long s = a[0] + a[1];
    long x = 0;
    for (int i = 2; i < n; i++) {
      x ^= a[i];
    }

    // a ^ b = x
    // a + b = s
    // a + a^x = s
    // a^a^x + a&(a^x) << 1 = s
    // a & (a^x) = (s-x)/2

    if ((s - x) < 0 || (s - x) % 2 != 0) {
      System.out.println(-1);
      return;
    }
    // a & b = d
    // a & a^x = d
    long d = (s - x) / 2;
    // x d a
    // 0 0 0
    // 1 0 any
    // 0 1 1
    // 1 1 ng

    if (d > a[0]) {
      System.out.println(-1);
      return;
    }

    long ret = d;
    for (int k = 63; k >= 0; k--) {
      long xb = x >> k & 1;
      long db = d >> k & 1;
      if (xb == 1 && db == 1) {
        System.out.println(-1);
        return;
      }

      if (xb == 1) {
        long z = 1L << k;

        if (ret + z <= a[0]) {
          ret += z;
        }
      }
    }
    ret = a[0] - ret;
    System.out.println(ret == a[0] ? -1 : ret);
  }

  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));
  }
}


Measured memory_bytes: 36680