public void solve() throws IOException {
    String s = nextLine();
    String t = nextLine();
    List<Integer> diff = new ArrayList<Integer>();
    for (int x = 0; x < s.length(); x++) {
      if (s.charAt(x) != t.charAt(x)) {
        diff.add(x);
      }
    }
    if (diff.size() % 2 != 0) {
      System.out.println("impossible");
    } else {
      char[] p = s.toCharArray();
      for (int x = 0; x < diff.size() / 2; x++) {
        int index = diff.get(x);
        p[index] = (p[index] == '1') ? '0' : '1';
      }
      System.out.println(new String(p));
    }
  }