#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define srep(i,s,t) for(int i = s; i < t; ++i)
#define drep(i,n) for(int i = (n)-1; i >= 0; --i)
using namespace std;
typedef long long int ll;
typedef pair<int,int> P;
#define yn {puts("Yes");}else{puts("No");}
#define MAX_N 200005

int main() {
    string s, t;
    cin >> s >> t;
    int n = s.size();
    int m = t.size();
    int need[n];
    rep(i,n)need[i] = -1;
    int now = m-1;
    drep(i,n){
        if(now == -1)break;
        while(now >= 0){
            if(t[now] == s[i]){
                need[i] = now;
                now--;
                break;
            }
            now--;
        }
    }

    if(need[0] >= 0){
        cout << -1 << endl;
        return 0;
    }

    int next[n][26];
    rep(i,n)rep(j,26)next[i][j] = n;
    drep(i,n-1){
        rep(j,26){
            next[i][j] = next[i+1][j];
        }
        int x = s[i+1] - 'a';
        next[i][x] = i+1;
    }
    int start[26];
    rep(j,26)start[j] = next[0][j];
    start[s[0]-'a'] = 0;

    string ans;
    now = 0;
    int ite = 0;
    int start_flag = 1;
    while(true){
        // cout << ite << endl;
        if(start_flag){
            start_flag = 0;
            rep(j,26){
                int nx = start[j];
                if(nx==n)continue;
                if(need[nx]<now){
                    char c = j + 'a';
                    ans += c;
                    if(now==m){
                        cout << ans << endl;
                        return 0;
                    }
                    while(now<m){
                        if(t[now] == c){
                            now++;
                            break;
                        }
                        now++;
                        if(now==m){
                            cout << ans << endl;
                            return 0;
                        }
                    }
                    ite = nx;
                    break;
                }
            }
        }else{
            rep(j,26){
                int nx = next[ite][j];
                if(nx==n)continue;
                if(need[nx]<now){
                    char c = j + 'a';
                    ans += c;
                    if(now==m){
                        cout << ans << endl;
                        return 0;
                    }
                    while(now<m){
                        if(t[now] == c){
                            now++;
                            break;
                        }
                        now++;
                        if(now==m){
                            cout << ans << endl;
                            return 0;
                        }
                    }
                    ite = nx;
                    break;
                }
            }
        }
    }

    return 0;
}