#include <iostream>
#include <algorithm>
#include <iomanip>
#include <vector>
#include <queue>
#include <set>
#include <map>

using namespace std;
typedef long long ll;

string S, T;

int substr_idx[100000];
int next_t[100005][26];
int next_s[100005][26];

int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout << setprecision(10) << fixed;
    cin >> S >> T;
    int cur_t = T.size()-1;
    for(int i = S.size()-1; i >= 0; i--){
        while(cur_t >= 0){
            if(T[cur_t] == S[i]) break;
            cur_t--;
        }
        substr_idx[i] = cur_t;
        if(cur_t >= 0) cur_t--;
    }
    if(substr_idx[0] != -1){
        cout << -1 << endl;
        return 0;
    }
    // T
    for(int i = 0; i < 26; i++){
        next_t[T.size()-1][i] = T.size();
    }
    next_t[T.size()-1][T[T.size()-1]-'a'] = T.size()-1;
    for(int i = T.size()-1; i >= 0; i--){
        if(i == T.size()-1) continue;
        for(int j = 0; j < 26; j++){
            next_t[i][j] = next_t[i+1][j];
        }
        next_t[i][T[i]-'a'] = i;
    }
    // S
    for(int i = 0; i < 26; i++){
        next_s[S.size()-1][i] = S.size();
    }
    next_s[S.size()-1][S[S.size()-1]-'a'] = S.size()-1;
    for(int i = S.size()-1; i >= 0; i--){
        if(i == S.size()-1) continue;
        for(int j = 0; j < 26; j++){
            next_s[i][j] = next_s[i+1][j];
        }
        next_s[i][S[i]-'a'] = i;
    }
    int cur_s = 0; cur_t = 0;
    
    string ans;
    while(true){
        if(cur_t > T.size()) break;
        if(cur_t >= T.size()) {
            for(int i = 0; i < 26; i++){
                char c = 'a'+i;
                int s = next_s[cur_s][i];
                if(s < S.size()){
                    ans += c;
                    break;
                }
            }
            break;
        }
        for(int i = 0; i < 26; i++){
            int s = next_s[cur_s][i];
            int t = next_t[cur_t][i];
            char c = 'a'+i;
            if(s < S.size() && t == T.size()) {
                cur_t = t+1;
                ans += c;
                break;
            }
            if(s < S.size() && substr_idx[s] < t){
                ans += c;
                cur_s = s+1;
                cur_t = t+1;
                break;
            }
        }
    }
    cout << ans << endl;
}