#line 1 "main.cpp"

/**
 * @title Template
 */

#include <iostream>
#include <algorithm>
#include <utility>
#include <numeric>
#include <vector>
#include <array>
#include <cassert>

#line 2 "/Users/kodamankod/Desktop/Programming/Library/other/chmin_chmax.cpp"

template <class T, class U>
constexpr bool chmin(T &lhs, const U &rhs) {
  if (lhs > rhs) { lhs = rhs; return true; }
  return false;
}

template <class T, class U>
constexpr bool chmax(T &lhs, const U &rhs) {
  if (lhs < rhs) { lhs = rhs; return true; }
  return false;
}

/**
 * @title Chmin/Chmax
 */
#line 2 "/Users/kodamankod/Desktop/Programming/Library/other/range.cpp"

#line 4 "/Users/kodamankod/Desktop/Programming/Library/other/range.cpp"

class range {
public:
  class iterator {
  private:
    int64_t M_position;

  public:
    constexpr iterator(int64_t position) noexcept: M_position(position) { }
    constexpr void operator ++ () noexcept { ++M_position; }
    constexpr bool operator != (iterator other) const noexcept { return M_position != other.M_position; }
    constexpr int64_t operator * () const noexcept { return M_position; }

  };

  class reverse_iterator {
  private:
    int64_t M_position;
  
  public:
    constexpr reverse_iterator(int64_t position) noexcept: M_position(position) { }
    constexpr void operator ++ () noexcept { --M_position; }
    constexpr bool operator != (reverse_iterator other) const noexcept { return M_position != other.M_position; }
    constexpr int64_t operator * () const noexcept { return M_position; }

  };
  
private:
  const iterator M_first, M_last;

public:
  constexpr range(int64_t first, int64_t last) noexcept: M_first(first), M_last(std::max(first, last)) { }
  constexpr iterator begin() const noexcept { return M_first; }
  constexpr iterator end() const noexcept { return M_last; }
  constexpr reverse_iterator rbegin() const noexcept { return reverse_iterator(*M_last - 1); } 
  constexpr reverse_iterator rend() const noexcept { return reverse_iterator(*M_first - 1); } 

};

/**
 * @title Range
 */
#line 2 "/Users/kodamankod/Desktop/Programming/Library/other/rev.cpp"

#include <type_traits>
#include <iterator>
#line 6 "/Users/kodamankod/Desktop/Programming/Library/other/rev.cpp"

template <class T>
class rev_impl {
public:
  using iterator = decltype(std::rbegin(std::declval<T>()));

private:
  const iterator M_begin;
  const iterator M_end;

public:
  constexpr rev_impl(T &&cont) noexcept: M_begin(std::rbegin(cont)), M_end(std::rend(cont)) { }
  constexpr iterator begin() const noexcept { return M_begin; }
  constexpr iterator end() const noexcept { return M_end; }

};

template <class T>
constexpr decltype(auto) rev(T &&cont) {
  return rev_impl<T>(std::forward<T>(cont));
}

/**
 * @title Reverser
 */
#line 17 "main.cpp"

using i32 = int32_t;
using i64 = int64_t;
using u32 = uint32_t;
using u64 = uint64_t;

constexpr i32 inf32 = (i32(1) << 30) - 1;
constexpr i64 inf64 = (i64(1) << 62) - 1;

std::vector<std::array<i32, 26>> calc(const std::string &S) {
  std::array<i32, 26> cur;
  cur.fill(inf32);
  const size_t N = S.size();
  std::vector<std::array<i32, 26>> res(N + 1);
  res.back() = cur;
  for (auto i: rev(range(0, N))) {
    cur[S[i] - 'a'] = i;
    res[i] = cur;
  }
  return res;
}

std::vector<i32> takes(std::string S, std::string T) {
  std::reverse(S.begin(), S.end());
  std::reverse(T.begin(), T.end());
  const size_t N = S.size();
  std::vector<i32> res(N, inf32);
  const auto next = calc(T);
  i32 cur = -1;
  for (auto i: range(0, N)) {
    if (cur == inf32) {
      break;
    }
    cur = next[cur + 1][S[i] - 'a'];
    chmin(res[i], cur + 1);
  }
  std::reverse(res.begin(), res.end());
  return res;
}

int main() {
  std::string S, T;
  std::cin >> S >> T;
  const auto from_back = takes(S, T);
  // for (auto x: from_back) {
  //   std::cout << x << ' ';
  // }
  // std::cout << '\n';
  // return 0;
  if (from_back.front() != inf32) {
    std::cout << "-1\n";
    return 0;
  }
  const auto nextS = calc(S);
  const auto nextT = calc(T);
  i32 posS = -1, posT = -1;
  std::string ans;
  while (true) {
    for (auto i: range(0, 26)) {
      i32 nposS = nextS[posS + 1][i];
      if (nposS == inf32) {
        continue;
      }
      i32 nposT = nextT[posT + 1][i];
      if (nposT == inf32) {
        ans += ('a' + i);
        std::cout << ans << '\n';
        return 0;
      }
      if (from_back[nposS] + nposT > S.size()) {
        ans += ('a' + i);
        posS = nposS;
        posT = nposT;
        break;
      }
    }
  }
  return 0;
}