#define _USE_MATH_DEFINES
#include <bits/stdc++.h>
using namespace std;
signed main() { 
  ios::sync_with_stdio(false); cin.tie(0);
  string s, t;
  cin >> s >> t;
  int n = s.size(), m = t.size();
  vector<int> ls(n + 1, -1);
  for (int i = n - 1, j = m - 1; i >= 0; i--) {
    while (j >= 0 && t[j] != s[i]) j--;
    ls[i] = j;
  }
  if (ls[0] != -1) {
    cout << -1 << endl;
    return 0;
  }
  vector<vector<int>> as(n + 1, vector<int>(26, -1));
  for (int i = n - 1; i >= 0; i--) {
    for (int j = 0; j < 26; j++) {
      if (j == s[i] - 'a') {
        as[i][j] = i;
      } else {
        if (i < n - 1) as[i][j] = as[i + 1][j];
      }
    }
  }
  vector<vector<int>> at(m + 1, vector<int>(26, -1));
  for (int i = m - 1; i >= 0; i--) {
    for (int j = 0; j < 26; j++) {
      if (j == t[i] - 'a') {
        at[i][j] = i;
      } else {
        if (i < m - 1) at[i][j] = at[i + 1][j];
      }
    }
  }
  string ans;
  int i = 0, j = 0;
  while (i < n) {
    for (int c = 0; c < 26; c++) {
      if (as[i][c] == -1) continue;
      if (at[j][c] == -1) {
        ans += c + 'a';
        goto done; 
      }
      int a = as[i][c];
      int b = at[j][c];
      if (ls[a] == -1 || ls[a] > b) {
        ans += c + 'a';
        i = a + 1;
        j = b + 1;
        break;
      }
    }
  }
  done:;
  cout << ans << endl;
  return 0;
}