#include<bits/stdc++.h>
#define ll long long
using namespace std;
int main(){ 
	ios::sync_with_stdio(0);
	cin.tie(0);
	cout.tie(0);
	string a;
	string b;
	char c1,c2;
	bool l=0;
	cin>>a>>b;
	int cnt=min(a.size(),b.size());
	for(int i=0;i<cnt;i++){
		if(a[i]<b[i]){
			c1=a[i];
			c2=b[i];
			break;
		}
		else if(a[i]>b[i]){
			l=1;
			break;
		}
	}
	if(l){
		cout<<"abcdefghijklmnopqrstuvwxyz";
	}
	else{
		cout<<c1<<c2;
		for(char i='a';i!='z';i++){
			if(i==c1||i==c2){
				continue;
			}
			cout<<i;
		}
	}
	return 0;
}