#include <bits/stdc++.h>   using namespace std;   vector<int> vec[10];   vector<vector<int> > vvv(183);   queue<int> qu;   map<string, int> mp;   map<string, int>::iterator iter;   bool ch[10] = {false};   int main() {     string s1, s2;     cin >> s1 >> s2;     for (int i = 0; i < s1.length(); i++) {       if (s1[i] >= s2[i])         s1[i] = s2[i];       else {         cout << -1 << endl;         return 0;       }     }     cout << s1 << endl;     return 0;   }   