#include <bits/stdc++.h>

using namespace std ;

const int MAX = 2e5 + 10 ;

int arr[MAX] ;
int n ;

string s , t ;
string s2 , t2 ;

int main()
{
	ios_base::sync_with_stdio(0) ;
	cin.tie(0) ;
	cin>>s ;
	cin>>t ;
	int n = s.size() , m = t.size() ;
	for(int i = 0 ; i < n ; ++i)
	{
		s2 = "" , t2 = "" ;
		int x = n - i , y = m + i ;
		if(y < x || (y - x) % 2 != 0)
			continue ;
		int diff = (y - x) / 2 ;
		for(int j = 0 ; j < diff ; ++j)
			s2.push_back(t[m-diff+j]) ;
		for(int j = i ; j < n ; ++j)
			s2.push_back(s[j]) ;
		for(int j = 0 ; j < m-diff ; ++j)
			t2.push_back(t[j]) ;
		for(int j = 0 ; j < i ; ++j)
			t2.push_back(s[j]) ;
		if(s2 == t2)
			return cout<<s2<<"\n" , 0 ;
	}
	return cout<<-1<<"\n" , 0 ;
}