//#include <atcoder/all>
#include <iostream> // cout, endl, cin
#include <string> // string, to_string, stoi
#include <vector> // vector
#include <algorithm> // min, max, swap, sort, reverse, lower_bound, upper_bound
#include <utility> // pair, make_pair
#include <tuple> // tuple, make_tuple
#include <cstdint> // int64_t, int*_t
#include <cstdio> // printf
#include <map> // map
#include <queue> // queue, priority_queue
#include <set> // set
#include <stack> // stack
#include <deque> // deque
#include <unordered_map> // unordered_map
#include <unordered_set> // unordered_set
#include <bitset> // bitset
#include <cctype> // isupper, islower, isdigit, toupper, tolower
#include <math.h>
#include <iomanip>
using namespace std;  
//using namespace atcoder;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define repi(i, a, b) for (int i = (int)(a); i < (int)(b); i++)
typedef long long ll;
typedef unsigned long long ull;
const ll inf=1e18;  
using graph = vector<vector<int> > ;
using P= pair<ll,ll>;
using vi=vector<int>;
using vvi=vector<vi>;
using vll=vector<ll>;
using vvll=vector<vll>;
using vp=vector<P>;
using vpp=vector<vp>;
//string T="ABCDEFGHIJKLMNOPQRSTUVWXYZ";
string S="abcdefghijklmnopqrstuvwxyz";
//g++ main.cpp -std=c++14 -I .  
//cout <<setprecision(20);
//cout << fixed << setprecision(10);
const double PI = acos(-1);

int vx[]={0,1,0,-1,-1,1,1,-1},vy[]={1,0,-1,0,1,1,-1,-1};
 
ll pow_pow(ll x,ll n,ll mod){
    if(n==0) return 1;
    x%=mod;
    ll res=pow_pow(x*x%mod,n/2,mod);
    if(n&1)res=res*x%mod;
    return res;
}
 
struct UnionFind {
    vector<int> par, siz;
    UnionFind(int n) : par(n, -1) , siz(n, 1) { }
    int root(int x) {
        if (par[x] == -1) return x;
        else return par[x] = root(par[x]);
    }
    bool issame(int x, int y) {
        return root(x) == root(y);
    }
    bool unite(int x, int y) {
        x = root(x), y = root(y);
        if (x == y) return false; 
        if (siz[x] < siz[y]) swap(x, y);
        par[y] = x;
        siz[x] += siz[y];
        return true;
    }
    int size(int x) {
        return siz[root(x)];
    }
};

int gcd(int x,int y){
    if(y==0)return x;
    return gcd(y,x%y);
}

ll lcm(ll x,ll y){
    return ll(x/gcd(x,y))*y;
}
 
 
 
 
template<class T> bool chmin(T& a, T b) {
    if (a > b) {
        a = b;
        return true;
    }
    else return false;
}
template<class T> bool chmax(T& a, T b) {
    if (a < b) {
        a = b;
        return true;
    }
    else return false;
}
 
// https://youtu.be/L8grWxBlIZ4?t=9858
// https://youtu.be/ERZuLAxZffQ?t=4807 : optimize
// https://youtu.be/8uowVvQ_-Mo?t=1329 : division
const ll mod =1e9+7;
struct mint {
  ll x; // typedef long long ll;
  mint(ll x=0):x((x%mod+mod)%mod){}
  mint operator-() const { return mint(-x);}
  mint& operator+=(const mint a) {
    if ((x += a.x) >= mod) x -= mod;
    return *this;
  }
  mint& operator-=(const mint a) {
    if ((x += mod-a.x) >= mod) x -= mod;
    return *this;
  }
  mint& operator*=(const mint a) { (x *= a.x) %= mod; return *this;}
  mint operator+(const mint a) const { return mint(*this) += a;}
  mint operator-(const mint a) const { return mint(*this) -= a;}
  mint operator*(const mint a) const { return mint(*this) *= a;}
  mint pow(ll t) const {
    if (!t) return 1;
    mint a = pow(t>>1);
    a *= a;
    if (t&1) a *= *this;
    return a;
  }
 
  // for prime mod
  mint inv() const { return pow(mod-2);}
  mint& operator/=(const mint a) { return *this *= a.inv();}
  mint operator/(const mint a) const { return mint(*this) /= a;}
};
istream& operator>>(istream& is, const mint& a) { return is >> a.x;}
ostream& operator<<(ostream& os, const mint& a) { return os << a.x;}
// combination mod prime
// https://www.youtube.com/watch?v=8uowVvQ_-Mo&feature=youtu.be&t=1619
struct combination {
  vector<mint> fact, ifact;
  combination(int n):fact(n+1),ifact(n+1) {
    //assert(n < mod);
    fact[0] = 1;
    for (int i = 1; i <= n; ++i) fact[i] = fact[i-1]*i;
    ifact[n] = fact[n].inv();
    for (int i = n; i >= 1; --i) ifact[i-1] = ifact[i]*i;
  }
  mint operator()(int n, int k) {
    if (k < 0 || k > n) return 0;
    return fact[n]*ifact[k]*ifact[n-k];
  }
  mint p(int n, int k) {
    return fact[n]*ifact[n-k];
  }
} c(10000050);
using vm=vector<mint> ;
using vvm=vector<vm> ;
 
 
ll sqrt_(ll x) {
  ll l = 0, r = ll(3e9)+1;
  while (l+1<r) {
    ll c = (l+r)/2;
    if (c*c <= x) l = c; else r = c;
  }
  return l;
}


int valid(int x,int y,int h,int w){
  if(x>=0 && y>=0 && x<h && y<w)return 1;
  else return 0;
}


 

ll comb[51][51];

void init_comb(){
  comb[0][0]=1;
  for(int i=1; i<=50; i++){
    for(int j=0; j<=i; j++){
      if(j==0 || j==i)comb[i][j]=1;
      else comb[i][j]=comb[i-1][j]+comb[i-1][j-1];
    }
  }
}

ll nCk(int n,int k){
  return comb[n][k];
}

struct edge{
  int to; ll w;
  edge(int to,ll w) : to(to),w(w) {}
}; 



int main(){
  string s,t; cin >> s >> t;
  s='@'+s;
  t='@'+t;
  int n=s.size(),m=t.size();
  vvi nexts(n,vi(26,-1)),nextt(m,vi(26,-1));
  vi id(26,-1);
  vi dp(n,-1);
  for(int i=n-1; i>=1; i--){
    nexts[i]=id;
    id[s[i]-'a']=i;
  }
  nexts[0]=id;
  rep(i,26)id[i]=-1;
  for(int i=m-1; i>=1; i--){
    nextt[i]=id;
    id[t[i]-'a']=i;
  }
  nextt[0]=id;
  int r=m-1;   
  for(int i=n-1; i>=0; i--){
    while(r>=0){
      if(s[i]==t[r]){
        dp[i]=r;
        r--;
        break;
      }
      r--;
    }
    if(dp[i]==-1)break;
  }
  if(dp[1]>=0){
    cout << -1 << endl;
    return 0;   
  }
  int index=0,index2=0;
  vi ans;
  while(1){
    rep(j,26){
      int flag=nexts[index][j];
      int ok=0;   
      if(flag!=-1){
        if(dp[flag]<=index2)ok=1;
      }
      if(ok){ 
        ans.push_back(j);
        index=flag;
        index2=nextt[index2][j];
        if(index2==-1){
           for(auto u:ans)cout << S[u];
           cout << endl;
           return 0;
        }
        break;
      }
    }  
  }
}