#include <iostream>
#include <string>
#include <vector>
using namespace std;
const int alphabet_size = 26;

int main() {
    string s, t;
    cin >> s >> t;
    
    int ss = s.size(), ts = t.size();
    vector<vector<int>> s_next(ss + 1, vector<int>(alphabet_size, ss));
    for (int i = ss - 1; i >= 0; --i) {
        for (int j = 0; j < alphabet_size; ++j) {
            s_next[i][j] = s_next[i + 1][j];
        }
        s_next[i][s[i] - 'a'] = i;
    }
    vector<vector<int>> t_next(ts + 1, vector<int>(alphabet_size, ts));
    for (int i = ts - 1; i >= 0; --i) {
        for (int j = 0; j < alphabet_size; ++j) {
            t_next[i][j] = t_next[i + 1][j];
        }
        t_next[i][t[i] - 'a'] = i;
    }
    
    vector<int> back_substr(ss + 1, ts);
    int k = ts;
    for (int i = ss - 1; i >= 0; --i) {
        while (k > 0) {
            --k;
            if (s[i] == t[k]) {
                back_substr[i] = k;
                break;
            }
        }
        if (back_substr[i] == ts) back_substr[i] = -1;
    }
    
    if (back_substr[0] != -1) {
        cout << -1 << endl;
        return 0;
    }
    
    string ans;
    int s_cnt = 0, t_cnt = 0;
    bool ended = false;
    while (!ended) {
        for (int i = 0; i < alphabet_size; ++i) {
            if (s_next[s_cnt][i] == ss) {
                continue;
            } else if (t_next[t_cnt][i] == ts) {
                ans.push_back('a' + i);
                ended = true;
                break;
            } else {
                if (back_substr[s_next[s_cnt][i] + 1] < t_next[t_cnt][i] + 1) {
                    s_cnt = s_next[s_cnt][i] + 1;
                    t_cnt = t_next[t_cnt][i] + 1;
                    ans.push_back('a' + i);
                    break;
                }
            }
        }
    }
    
    cout << ans << endl;
    return 0;
}