#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;
void solvesmall(){
	set<string>setS,setT;
    rep(i,1<<len(S)){
        string now;
        rep(j,len(S)){
            if(i>>j&1)now+=S[j];
        }
        setS.insert(now);
    }
    rep(i,1<<len(T)){
        string now;
        rep(j,len(T)){
            if(i>>j&1)now+=T[j];
        }
        setT.insert(now);
    }
    for(string s:setS){
        if(setT.find(s)==setT.end()){
            cout<<s<<endl;return;
        }
    }
    cout<<"-1"<<endl;
}
signed main(){
	cin>>S>>T;
	assert(len(S)<=100000&&len(T)<=100000);
	if(len(S)<=18&&len(T)<=18)solvesmall();
	else {
		int cnt1=0,cnt2=0;
		rep(i,len(S))cnt1+=S[i]=='a';
		rep(i,len(T))cnt2+=S[i]=='a';
		if(cnt1<=cnt2){
			cout<<"-1"<<endl;
		}else {
			rep(i,cnt2+1)cout<<'a';
			cout<<endl;
		}
	}
}