#include <string>
#include <vector>
#include <iostream>
#include <algorithm>
using namespace std;
const int inf = 1012345678;
vector<vector<int> > calc_next(string S) {
	int N = S.size();
	vector<vector<int> > dp(N + 1, vector<int>(26, inf));
	for (int i = N - 1; i >= 0; --i) {
		dp[i] = dp[i + 1];
		dp[i][S[i] - 'a'] = i;
	}
	return dp;
}
int main() {
	string S, T;
	cin >> S >> T;
	int LS = S.size(), LT = T.size();
	vector<int> dp(LS + 1); dp[LS] = LT;
	int ptr = LS;
	for (int i = LS - 1; i >= 0; --i) {
		while (ptr != -1) {
			--ptr;
			if (ptr != -1 && S[i] == T[ptr]) break;
		}
		dp[i] = ptr;
	}
	vector<vector<int> > ns = calc_next(S);
	vector<vector<int> > nt = calc_next(T);
	string ans;
	int posa = 0, posb = 0;
	while (true) {
		bool found = false;
		for (int i = 0; i < 26; ++i) {
			if (ns[posa][i] != inf && (posb == inf || dp[ns[posa][i] + 1] < nt[posb][i])) {
				posa = ns[posa][i] + 1;
				posb = min(nt[posb][i] + 1, inf);
				ans += char('a' + i);
				found = true;
				break;
			}
		}
		if (!found) break;
	}
	if (ans.empty()) cout << -1 << endl;
	else cout << ans << endl;
	return 0;
}