#include<bits/stdc++.h>
using namespace std;
#define int long long
#define rep(i,n) for(int i=0;i<n;i++)
#define REP(i,n) for(int i=1;i<n;i++)
#define rev(i,n) for(int i=n-1;i>=0;i--)
#define all(v) v.begin(),v.end()
#define P pair<int,int>
#define len(s) (int)s.size()

template<class T> inline bool chmin(T &a, T b){
	if(a>b){a=b;return true;}
	return false;
}
template<class T> inline bool chmax(T &a, T b){
	if(a<b){a=b;return true;}
	return false;
}
constexpr int mod = 1e9+7;
constexpr int inf = 3e18;

string S,T;
int nexS[100005][26],nexT[100005][26];
int inc[100005];
signed main(){
	cin>>S>>T;
	assert(len(S)<=100000&&len(T)<=100000);
	rep(i,26){
		nexS[len(S)][i]=len(S);
		nexT[len(T)][i]=len(T);
	}
	rev(i,len(S)){
		rep(j,26)nexS[i][j]=nexS[i+1][j];
		nexS[i][S[i]-'a']=i;
	}
	rev(i,len(T)){
		rep(j,26)nexT[i][j]=nexT[i+1][j];
		nexT[i][T[i]-'a']=i;
	}
	int now=len(T);inc[len(S)]=now;
	rev(i,len(S)){
		now=max(-1ll,now-1);
		while(now>=0&&T[now]!=S[i])now--;
		inc[i]=now;
	}
	if(inc[0]!=-1){
		cout<<"-1\n";return 0;
	}
	string Ans;
	int curS=0,curT=0;
	while(curS<len(S)){
		rep(i,26){
			if(nexS[curS][i]==len(S))continue;
			if(nexT[curT][i]==len(T)){
				Ans+=('a'+i);
				cout<<Ans<<endl;return 0;
			}
			if(inc[nexS[curS][i]+1]>=nexT[curT][i]+1)continue;
			curS=nexS[curS][i]+1;
			curT=nexT[curT][i]+1;
			Ans+=('a'+i);
			break;
		}
	}
}