#include <bits/stdc++.h>
using namespace std;
#define modulo 998244353
#define mod(mod_x) ((((long long)mod_x+modulo))%modulo)
#define Inf 1000000005

int main(){
	
	string S,T;
	cin>>S>>T;

	vector<vector<int>> nS(S.size()+1),nT(T.size()+1);
	{
		vector<int> now(26,Inf);
		for(int i=S.size();i>=0;i--){
			nS[i] = now;
			if(i!=0){
				now[S[i-1]-'a']=i;
			}
		}
	}
	{
		vector<int> now(26,Inf);
		for(int i=T.size();i>=0;i--){
			nT[i] = now;
			if(i!=0){
				now[T[i-1]-'a']=i;
			}
		}
	}
	
	vector<int> ind(T.size(),Inf);
	int pos = S.size();
	for(int i=T.size()-1;i>=0;i--){
		if(pos!=0&&T[i]==S[pos-1]){
			pos--;
		}
		ind[i] = pos;
	}
	
	string ans = "";
	int posS = 0,posT = 0;
	while(true){
		bool f = false;
		for(int i=0;i<26;i++){
			if(nS[posS][i]==Inf)continue;
			if(nT[posT][i]==Inf){
				ans += 'a'+i;
				goto L;
			}
			int s = nS[posS][i]-1;
			int t = nT[posT][i]-1;
			if(ind[t]<=s)continue;
			posS = s+1;
			posT = t+1;
			ans += 'a'+i;
			f=true;
			break;
		}
		if(!f)break;
	}
	
	L:;
	
	if(ans==""){
		cout<<-1<<endl;
	}
	else cout<<ans<<endl;
	
	return 0;
}