batmend1

#include<iostream>
using namespace std;

string x , y, p;

int main(){

	cin >> x >> y;
	int n = x.size();

	for( int i = 0 ; i < n ; i ++ ){
		if( y[i] > x[i] ){
			cout << -1 ;
			return 0;
		}
		else{
			if( x[i] == y[i] ) p += 'z'; // end uurchlult hiisen shu. string-d char nemehed shuud += geed char-aa zaaj ugnu.
			else p += y[i]; // herev olon char esul string zalgaval aldaa zaana shu.
		}
	}

	cout << p << endl; // string-g for bichih shaardlagagui shuud hevlej bolno.

}