#include<bits/stdc++.h>

#define LL long long
#define pb push_back
#define F first
#define S second
#define all(x) x.begin(),x.end()

const double eps=1e-7,PI=3.1415926;
const int N=1e6+10;

using namespace std;

int n,q,m,k,x,y,a[N],mx=-1,mn=1e9,sum;
string str1,str2,s1,s2;
map < int , int > mp;
vector < int > vec;

int main(){
    cin>>s1>>s2;
    n=s1.size();
    m=s2.size();
    if (s1==s2)return cout<<s1<<endl,0;
    if ((n+m)%2)return cout<<-1<<endl,0;
    for (int i=0;i<n;i++){
        str1+=s1[i];
        string tmp="",tmp2=s2;
        bool b=1;
        while(tmp.size()+str1.size()<(n+m)/2){
            if (tmp2.empty()){
                b=0;
                break;
            }
            tmp.pb(tmp2.back());
            tmp2.pop_back();
        }
        reverse(all(tmp));
        tmp+=str1;
        if (b==0)continue;
        string ch=tmp2;
        ch+=s1.substr(i+1,n-i-1);
        if (tmp==ch)return cout<<tmp<<endl,0;
    }
    str1="";
    string tmp="",tmp2=s2;
    bool b=1;
    while(tmp.size()<(n+m)/2){
        if (tmp2.empty()){
            b=0;
            break;
        }
        tmp.pb(tmp2.back());
        tmp2.pop_back();
    }
    reverse(all(tmp));
    if (b){
        string ch=tmp2;
        ch+=s1;
        if (tmp==ch)return cout<<tmp<<endl,0;
    }
    cout<<-1<<endl;
    return 0;
}