#include <bits/stdc++.h>
using namespace std;

int main() {
    ios_base::sync_with_stdio(0);
    cin.tie(0);

    string a, b;
    cin >> a >> b;
    list<char> a1, a2, b1, b2;
    int n = a.size();

    if(a.size() != b.size())
        return cout << "-1\n", 0;
    
    for(int i = 0; i < n; i ++)
        a2.push_back(a[i]), b1.push_back(b[i]);

    for(int i = 0; i < n; i ++){
        a1.push_back(a[i]);
        a2.pop_front();
        b1.pop_back();
        b2.push_front(b[n - i - 1]);

        string A, B;
        for(char x : b2)
            A.push_back(x);
        for(char x : a2)
            A.push_back(x);
        
        for(char x : b1)
            B.push_back(x);
        for(char x : a1)
            B.push_back(x);
        
        if(A == B)
            return cout << A << '\n', 0;
    }
    cout << "-1\n";
}