#include <iostream>
#include <map>
#include <set>
#include <queue>
#include <cmath>
#include <vector>
#include <string>
#include <algorithm>
#include <functional>
using namespace std;
#pragma warning (disable: 4996)

string S, T;
int cs[100009][26];
int ct[100009][26];
int dp[100009];

int main() {
	// Step #1. 入力
	cin >> S >> T;

	// Step #2. S について計算
	for (int i = 0; i < 26; i++) cs[S.size()][i] = S.size() + 2;
	for (int i = S.size() - 1; i >= 0; i--) {
		for (int j = 0; j < 26; j++) cs[i][j] = cs[i + 1][j];
		cs[i][S[i] - 'a'] = i;
	}

	// Step #3. T について計算
	for (int i = 0; i < 26; i++) ct[T.size()][i] = T.size() + 2;
	for (int i = T.size() - 1; i >= 0; i--) {
		for (int j = 0; j < 26; j++) ct[i][j] = ct[i + 1][j];
		ct[i][T[i] - 'a'] = i;
	}

	// Step #4. 貪欲法
	dp[S.size()] = T.size(); int cx = T.size() - 1;
	for (int i = S.size() - 1; i >= 0; i--) {
		while (cx >= 0 && T[cx] != S[i]) cx--;
		dp[i] = cx; cx--;
	}

	// Step #5. 辞書順貪欲
	int px = 0, qx = 0;
	string str = "";
	while (qx <= T.size()) {
		bool flag = false;
		for (int i = 0; i < 26; i++) {
			int nex1 = cs[px][i];
			int nex2 = ct[qx][i];
			if (nex1 == S.size() + 2) continue;
			if (dp[nex1 + 1] < nex2 + 1) {
				flag = true;
				str += ('a' + i);
				px = nex1 + 1;
				qx = nex2 + 1;
				break;
			}
		}
		if (flag == false) break;
	}

	// Step #6. 出力
	if (str == "") cout << "-1" << endl;
	else cout << str << endl;
	return 0;
}