#define _USE_MATH_DEFINES
#include <bits/stdc++.h>
using namespace std;
#define FOR(i,m,n) for(int i=(m);i<(n);++i)
#define REP(i,n) FOR(i,0,n)
#define ALL(v) (v).begin(),(v).end()
using ll = long long;
const int INF = 0x3f3f3f3f;
const ll LINF = 0x3f3f3f3f3f3f3f3fLL;
const double EPS = 1e-8;
const int MOD = 1000000007;
// const int MOD = 998244353;
const int dy[] = {1, 0, -1, 0}, dx[] = {0, -1, 0, 1};
const int dy8[] = {1, 1, 0, -1, -1, -1, 0, 1}, dx8[] = {0, -1, -1, -1, 0, 1, 1, 1};
template <typename T, typename U> inline bool chmax(T &a, U b) { return a < b ? (a = b, true) : false; }
template <typename T, typename U> inline bool chmin(T &a, U b) { return a > b ? (a = b, true) : false; }
struct IOSetup {
  IOSetup() {
    cin.tie(nullptr);
    ios_base::sync_with_stdio(false);
    cout << fixed << setprecision(20);
  }
} iosetup;

vector<vector<int>> nx_init(const string &s, const char basis = 'a', const int char_sz = 26) {
  int n = s.size();
  vector<vector<int>> nx(n, vector<int>(char_sz, n));
  nx[n - 1][s[n - 1] - basis] = n - 1;
  for (int i = n - 2; i >= 0; --i) {
    REP(j, char_sz) nx[i][j] = nx[i + 1][j];
    nx[i][s[i] - basis] = i;
  }
  return nx;
}

int main() {
  string s, t; cin >> s >> t;
  int n = s.length(), m = t.length();
  auto nx_s = nx_init(s), nx_t = nx_init(t);
  reverse(ALL(t));
  auto rev_nx_t = nx_init(t);
  vector<int> sub(n);
  sub[n - 1] = rev_nx_t[0][s[n - 1] - 'a'];
  for (int i = n - 2; i >= 0; --i) {
    sub[i] = sub[i + 1] >= m - 1 ? m : rev_nx_t[sub[i + 1] + 1][s[i] - 'a'];
  }
  REP(i, n) sub[i] = m - 1 - sub[i];
  if (sub[0] != -1) {
    cout << "-1\n";
    return 0;
  }
  string ans = "";
  int pos_s = -1, pos_t = -1;
  while (true) {
    REP(c, 26) {
      int nx_pos_s = nx_s[pos_s + 1][c];
      if (nx_pos_s == n) continue;
      if (pos_t + 1 == m || nx_t[pos_t + 1][c] == m) {
        ans += 'a' + c;
        cout << ans << '\n';
        return 0;
      }
      int nx_pos_t = nx_t[pos_t + 1][c];
      if (nx_pos_s + 1 < n && sub[nx_pos_s + 1] < nx_pos_t + 1) {
        ans += 'a' + c;
        pos_s = nx_pos_s;
        pos_t = nx_pos_t;
        break;
      }
    }
  }
}