#include <iostream>
#include <algorithm>
#include <iomanip>
#include <bits/stdc++.h>
#include <cstdio>
#include <string>
#define ll long long
#define ST(a) a.first
#define ND(a) a.second
#define fin(a,n) for(int i = a; i < n; i++)
#define fjn(a,n) for(int j = a; j < n; j++)
#define all(a) a.begin(),a.end()
#define FIFO ios_base::sync_with_stdio(false), cin.tie(nullptr), cout.tie(nullptr)
using namespace std;
void solve()
{
string s, t, ans = "";
cin >> s >> t;
bool found = true;
fin(0,s.size()){
if(s[i] < t[i]){
found = false; break;
}
}
if(!found){
cout << "-1\n"; return;
}
cout << t << '\n';
}
int main()
{
FIFO;
int tt = 1; //cin >> tt;
while(tt--){
solve();
}
return 0;
}
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