//#define _GLIBCXX_DEBUG

#include<bits/stdc++.h>
using namespace std;

#define endl '\n'
#define lfs cout<<fixed<<setprecision(10)
#define ALL(a)  (a).begin(),(a).end()
#define ALLR(a)  (a).rbegin(),(a).rend()
#define spa << " " <<
#define fi first
#define se second
#define MP make_pair
#define MT make_tuple
#define PB push_back
#define EB emplace_back
#define rep(i,n,m) for(ll i = (n); i < (ll)(m); i++)
#define rrep(i,n,m) for(ll i = (ll)(m) - 1; i >= (ll)(n); i--)
using ll = long long;
using ld = long double;
const ll MOD1 = 1e9+7;
const ll MOD9 = 998244353;
const ll INF = 1e18;
using P = pair<ll, ll>;
template<typename T1, typename T2>bool chmin(T1 &a,T2 b){if(a>b){a=b;return true;}else return false;}
template<typename T1, typename T2>bool chmax(T1 &a,T2 b){if(a<b){a=b;return true;}else return false;}
ll median(ll a,ll b, ll c){return a+b+c-max({a,b,c})-min({a,b,c});}
void ans1(bool x){if(x) cout<<"Yes"<<endl;else cout<<"No"<<endl;}
void ans2(bool x){if(x) cout<<"YES"<<endl;else cout<<"NO"<<endl;}
void ans3(bool x){if(x) cout<<"Yay!"<<endl;else cout<<":("<<endl;}
template<typename T1,typename T2>void ans(bool x,T1 y,T2 z){if(x)cout<<y<<endl;else cout<<z<<endl;}  
template<typename T>void debug(vector<vector<T>>&v,ll h,ll w){for(ll i=0;i<h;i++){cout<<v[i][0];for(ll j=1;j<w;j++)cout spa v[i][j];cout<<endl;}};
void debug(vector<string>&v,ll h,ll w){for(ll i=0;i<h;i++){for(ll j=0;j<w;j++)cout<<v[i][j];cout<<endl;}};
template<typename T>void debug(vector<T>&v,ll n){if(n!=0)cout<<v[0];for(ll i=1;i<n;i++)cout spa v[i];cout<<endl;};
template<typename T>vector<vector<T>>vec(ll x, ll y, T w){vector<vector<T>>v(x,vector<T>(y,w));return v;}
ll gcd(ll x,ll y){ll r;while(y!=0&&(r=x%y)!=0){x=y;y=r;}return y==0?x:y;}
vector<ll>dx={1,-1,0,0,1,1,-1,-1};vector<ll>dy={0,0,1,-1,1,-1,1,-1};
template<typename T>vector<T> make_v(size_t a,T b){return vector<T>(a,b);}
template<typename... Ts>auto make_v(size_t a,Ts... ts){return vector<decltype(make_v(ts...))>(a,make_v(ts...));}
template<typename T1, typename T2>ostream &operator<<(ostream &os, const pair<T1, T2>&p){return os << p.first << " " << p.second;}
template<typename T>ostream &operator<<(ostream &os, const vector<T> &v){for(auto &z:v)os << z << " ";cout<<"|"; return os;}
//mt19937 mt(chrono::steady_clock::now().time_since_epoch().count());
vector<vector<ll>>next_index(string s, ll start, ll kind, bool is_loop = false){ 
  //next[i][j]i文字目(0-indexed)以降で次に文字jが出てくるindex
  ll n=s.size();
  vector<vector<ll>>next(n+1,vector<ll>(kind));
  vector<ll> p(kind,n);
  rrep(i,0,n){
    rep(j,0,kind)next[i+1][j]=p[j];
    p[s[i]-start]=i;
  }
  if(is_loop){
    rrep(i,0,n){
    rep(j,0,kind)next[i+1][j]=p[j];
    p[s[i]-start]=i;
    }
  }
  rep(i,0,kind)next[0][i]=p[i];
  return next;
}
int main(){
  cin.tie(nullptr);
  ios_base::sync_with_stdio(false);
  ll res=0,buf=0;
  bool judge = true;
  string s,t;cin>>s>>t;
  ll n=s.size();
  ll m=t.size();
  vector<ll>suf(n+1,m);
  ll idx=m-1;
  rrep(i,0,n){
    while(idx>=0&&s[i]!=t[idx])idx--;
    suf[i]=idx;
    if(idx>=0)idx--;
  }
  if(suf[0]>=0){
    cout<<-1<<endl;
    return 0;
  }
  auto ns=next_index(s,'a',26);
  auto nt=next_index(t,'a',26);
  ll nows=0,nowt=0;
  string ret;
  while(nowt<=m){
    rep(i,0,26){
      if(suf[ns[nows][i]]<nowt){
        nows=ns[nows][i]+1;
        nowt=nt[nowt][i]+1;
        ret+='a'+i;
        break;
      }
    }
  }
  cout<<ret<<endl;
  return 0;
}