/**
 *    author:  otera    
**/
#include<iostream>
#include<string> 
#include<cstdio>
#include<cstring>
#include<vector>
#include<cmath>
#include<algorithm> 
#include<functional>
#include<iomanip>
#include<queue>
#include<deque>
#include<ciso646>
#include<random>
#include<map>
#include<set>
#include<complex>
#include<bitset>
#include<stack>
#include<unordered_map>
#include<unordered_set>
#include<utility>
#include<cassert>
using namespace std;

//#define int long long
typedef long long ll;
typedef unsigned long long ul;
typedef unsigned int ui;
typedef long double ld;
const int inf=1e9+7;
const ll INF=1LL<<60 ;
const ll mod=1e9+7 ;
#define rep(i,n) for(int i=0;i<n;i++)
#define per(i,n) for(int i=n-1;i>=0;i--)
#define Rep(i,sta,n) for(int i=sta;i<n;i++)
#define rep1(i,n) for(int i=1;i<=n;i++)
#define per1(i,n) for(int i=n;i>=1;i--)
#define Rep1(i,sta,n) for(int i=sta;i<=n;i++)
typedef complex<ld> Point;
const ld eps = 1e-8;
const ld pi = acos(-1.0);
typedef pair<int, int> P;
typedef pair<ld, ld> LDP;
typedef pair<ll, ll> LP;
#define fr first
#define sc second
#define all(c) c.begin(),c.end()
#define pb push_back
#define debug(x)  cerr << #x << " = " << (x) << endl;
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }

vector<vector<int>> calc_nxt(string s) {
    int n = (int)s.size();
    vector<vector<int>> res(n + 1, vector<int>(26, n));
    for(int i = n - 1; i >= 0; -- i) {
        rep(j, 26) res[i][j] = res[i + 1][j];
        res[i][s[i] - 'a'] = i;
    }
    return res;
}

void solve() {
	string s, t; cin >> s >> t;
    int n = (int)s.size(), m = (int)t.size();
    auto nxts = calc_nxt(s);
    auto nxtt = calc_nxt(t);
    vector<int> sub(n + 1, -1);
    sub[n] = m;
    int j = m - 1;
    for(int i = n - 1; i >= 0; -- i) {
        while(j >= 0 and s[i] != t[j]) -- j;
        if(j != -1) sub[i] = j, -- j;
    }
    if(sub[0] != -1) {
        cout << -1 << endl;
        return;
    }
    string ans = "";
    int i = 0;
    j = 0;
    while(1) {
        rep(k, 26) {
            if(nxts[i][k] == n) continue;
            else if(nxtt[j][k] == m) {
                ans += (char)('a' + k);
                cout << ans << endl;
                return;
            } else {
                int ni = nxts[i][k] + 1, nj = nxtt[j][k] + 1;
                if(sub[ni] >= nj) continue;
                ans += (char)('a' + k);
                i = ni, j = nj;
            }
        }
    }
}

signed main() {
	ios::sync_with_stdio(false);
	cin.tie(0);
	//cout << fixed << setprecision(10);
	//int t; cin >> t; rep(i, t)solve();
	solve();
    return 0;
}