#include <bits/stdc++.h>
using namespace std;
int INF = 1000000;
int main(){
  string S;
  cin >> S;
  string T;
  cin >> T;
  int N = S.size();
  int M = T.size();
  vector<int> p(N + 1, -1);
  p[N] = M;
  int ps = M - 1;
  for (int i = N - 1; i >= 0; i--){
    ps--;
    if (ps != -1){
      while (T[ps] != S[i]){
        ps--;
        if (ps == -1){
          break;
        }
      }
    }
    if (ps == -1){
      break;
    } else {
      p[i] = ps;
    }
  }
  if (p[0] != -1){
    cout << -1 << endl;
  } else {
    vector<vector<int>> next1(N + 1, vector<int>(26, N));
    for (int i = N - 1; i >= 0; i--){
      next1[i] = next1[i + 1];
      next1[i][S[i] - 'a'] = i;
    }
    vector<vector<int>> next2(M + 1, vector<int>(26, M));
    for (int i = M - 1; i >= 0; i--){
      next2[i] = next2[i + 1];
      next2[i][T[i] - 'a'] = i;
    }
    string ans = "";
    int p1 = 0, p2 = 0;
    while (1){
      bool ok = false;
      bool ok2 = false;
      for (int i = 0; i < 26; i++){
        if (next1[p1][i] != N){
          if (next2[p2][i] == M){
            ans += 'a' + i;
            ok = true;
            break;
          } else {
            int p3 = next1[p1][i] + 1;
            int p4 = next2[p2][i] + 1;
            if (p[p3] < p4){
              ans += 'a' + i;
              p1 = p3;
              p2 = p4;
              ok2 = true;
              break;
            }
          }
        }
      }
      if (ok || !ok2 || p1 == N){
        break;
      }
    }
    cout << ans << endl;
  }
}