#include "bits/stdc++.h"

#pragma GCC optimize("Ofast")
#pragma GCC optimize("-O2")

using namespace std;
using ll = long long;
using ld = long double;

int dx[] = {1, -1, 0, 0, -1, -1, 1, 1};
int dy[] = {0, 0, 1, -1, -1, 1, -1, 1};

const int N = 2E5 + 5, M = 2E6 + 10, MOD = 1E9 + 7, OO = 0x3f3f3f3f;;
int n, m, x;
string s, t;

int main() {
    ios_base::sync_with_stdio(0), cin.tie(0);
#ifndef ONLINE_JUDGE
    freopen("input.txt", "r", stdin);
#endif /*ONLINE_JUDGE*/

    cin >> s >> t;
    n = s.size(), m = t.size();

    for (int i = 0; i < n/2; ++i) {
        if(s.substr(0, i+1) == s.substr(n-i-1, i+1)){
            x = n - 2 * s.substr(0, i+1).size();
            if((m - x) & 1) continue;
            if(t.substr(0, (m-x)/2) == t.substr(m-(m-x)/2, (m-x)/2+1)){
                if(x == 0 || s.substr(i+1, x) == t.substr((m-x)/2, x))
                    return cout << t.substr(0, (m-x)/2) + t.substr((m-x)/2, x) + s.substr(0, i+1), 0;
            }
        }
    }

    cout << -1;


    return 0;
}