#define Print 1
#include <iostream>
#include <complex>
#include <iomanip>
#include <algorithm>
#include <queue>
#include <stack>
#include <string>
#include <vector>
#include <cmath>
#include <map>
#include <set>
#include <string.h>
#include <stdlib.h>
#include <assert.h>
#include<ext/pb_ds/assoc_container.hpp>
#include<ext/pb_ds/tree_policy.hpp> 

using namespace std;
using namespace __gnu_pbds;
template <typename T>
using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
// vector push_back push front top empty pop make_pair long long insert begin end  
#define int long long
typedef long long ll;
typedef vector<int> vi;
typedef vector<pair <int,int> > vpi;
typedef vector<long long> vll;
typedef pair<int,int> pi;
typedef pair<ll,ll> pll;
void display(vi &a){for(int z : a)cout << z << " ";cout << endl;}
#ifdef Print
    #define trace(x) cerr<<__FUNCTION__<<":"<<__LINE__<<": "#x" = "<<x<<endl;
#else
    #define trace(x);
#endif
#define F first
#define ln '\n'
#define S second
#define PB push_back
#define MP make_pair
#define B begin()
#define RB rbegin()
#define E end()
#define RE rend()
#define Z size()
#define REP(i,a,b) for (int i = a; i < b; i++)
#define L length()
#define show(a) cerr << " *** " << a << endl;
#define show1(a) cerr << " /// " << a << endl;
#define valid(a,b,c) (a >= b && a < c ? 1 : 0)
int dx[4] = {0,0,1,-1};
int dy[4] = {1,-1,0,0};
const int mod = (int)1e9 + 7;
const ll INF64 = 3e18;
void smxl(ll &a, ll b){if (a < b)a=b;}
void smnl(ll &a, ll b){if (a > b)a=b;}
void adsl(ll &a, ll b){a += b;if (a >= mod)a -= mod;}
void misl(ll &a, ll b){a -= b;if (a >= mod)a -= mod; if (a < 0)a += mod;}
void smx(ll &a, ll b){if (a < b)a=b;}
void smn(ll &a, ll b){if (a > b)a=b;}
void ads(ll &a, ll b){a += b;if (a >= mod)a -= mod;}
void mis(ll &a, ll b){a -= b;if (a >= mod)a -= mod; if (a < 0)a += mod;}
ll gcd(ll a, ll b) {return (b==0? a:gcd(b,a%b));}
ll egcd(ll a, ll b, ll & x, ll & y) {if (a == 0){x = 0;y = 1;return b;}ll x1, y1;ll d = egcd(b % a, a, x1, y1);x = y1 - (b / a) * x1;y = x1;return d;}
ll mbinp(ll a, ll b){a %= mod;if (b == 0)return 1;ll ans = mbinp(a, b/2);ll tmp = (ans * ans) % mod;if (b % 2)return ((tmp * a) % mod);return ((tmp) % mod);}
ll binp(ll a, ll b){if (b == 0)return 1;ll ans = binp(a, b/2);ll tmp = (ans * ans);if (b % 2)return ((tmp * a));return ((tmp));}
long long C(int n, int m){ll ret=1;for(int i=1;i<=m;i++){ret*=(n-i+1);ret/=i;}return ret;}
long long overbinp(long long a, int b){long long res = 1;while (b){if (b & 1){if (res < INF64 / a) res *= a;else return INF64;}if (b > 1){if (a < INF64 / a) a *= a;else return INF64;}b >>= 1;}return res;}

//double triarea(double a, double b, double c){double p= (a+b+c)/2;return (sqrt(p*(p-a)*(p-b)*(p-c)));}
//ll diag(ll n){return ((n*(n-1))/2 - n);}
//*find_by_order
//order_of_key
//number of divs of n=p^a*q^b*r^c  == (a+1)*(b+1)*(c+1)
//sum of divs of n=p^a*q^b*r^c  == (p^(a+1)-1)/(p-1) * (q^(b+1)-1)/(q-1) * (r^(c+1)-1)/(r-1)
//prod of divs of n=p^a*q^b*r^c  == n ^ (((a+1)*(b+1)*(c+1))/2)
vi kmp(string str)
{
    vi out;
    out.assign(str.L,0);
    int i = 1;
    int pos = 0;
    while(i < str.L)
    {
        if (str[i]==str[pos])
        {
            pos++;
            out[i]=pos;
            i++;
        }
        else
        {
            if (pos !=0)
            {
                pos = out[pos-1];
            }
            else
            {
                out[i] = 0;
                i++;
            }
        }
    }
    return out;
}


signed main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    cout.tie(NULL);
    string str,ptr;
    cin>>str>>ptr;
    if(str== ptr)
    {
        cout << str;
        return 0;
    }
    else
    {
        int i = 0;
        map<string,int> mp;
        while(i < str.L-1-i)
        {
            string t,x;
            REP(j,0,i+1)
                t += str[j];
            REP(j,str.L-1-i,str.L)
                x  += str[j];
            if (t == x)
            {
            string o;
            REP(j,i+1,str.L-1-i)
                o += str[j];
            mp[o] = i;
            }   
            i++;
        }
        i =0;
        while(i < ptr.L -1-i)
        {
            string t,x;
            REP(j,0,i+1)
                t += ptr[j];
            REP(j,ptr.L-1-i,ptr.L)
                x  += ptr[j];
            if (t == x)
            {
                string o;
                REP(j,i+1,ptr.L-1-i)
                    o += ptr[j];
                if (mp.count(o))
                {
                    REP(j,0,i+1)
                        cout << ptr[j];
                    cout << o;
                    REP(j,0,mp[o]+1)
                        cout << str[j];
                    return 0;
                }
            }
            i++;
        }
        cout << -1;
    }
}