#include <bits/extc++.h>

#ifndef DUMP
#define DUMP(...) (void)0
#endif

using namespace std;

int main() {
  cin.tie(nullptr)->sync_with_stdio(false);
  string s, t;
  cin >> s >> t;
  int n = size(s), m = size(t);
  vector<int> p(n + 1);
  p[n] = m;
  for (int i = n; i--;) {
    p[i] = p[i + 1] - 1;
    while (p[i] >= 0 and t[p[i]] != s[i]) --p[i];
  }
  if (p[0] >= 0) {
    cout << "-1\n";
    exit(0);
  }
  vector<array<int, 26>> nxt(n + 1);
  nxt[n].fill(n);
  for (int i = n; i--;) {
    nxt[i] = nxt[i + 1];
    nxt[i][s[i] - 'a'] = i;
  }
  string res;
  int i = 0, j = 0;
  while (true) {
    DUMP(i, j);
    for (char c = 'a'; c <= 'z'; ++c) {
      if (p[nxt[i][c - 'a']] >= j) continue;
      res += c;
      i = nxt[i][c - 'a'] + 1;
      while (j < m and t[j] != c) ++j;
      if (j == m) {
        cout << res << '\n';
        exit(0);
      }
      ++j;
      break;
    }
  }
  cout << res << '\n';
}