#include<bits/stdc++.h>
#include <stdio.h>
#include <algorithm>

#define all(x) x.begin(),x.end()
#define sc(x) scanf("%d",&x)
#define scl(x) scanf("%lld",&x)
#define LL long long
#define LD long double
#define pb push_back
#define F first
#define S second


const double PI=3.1415926535897932384626433;
const int KL=1e6;
const LL MOD=1e9+7;


using namespace std;
/*
typedef complex<double> point;
#define x real()
#define y imag()
*/

LL q,x,y,z,n,m,a[KL],mn=1e9,mx=-1e9,ans;
vector <LL> vec;
set <LL> s;
map <LL,LL> mp;


int main()
{

    string s,t;
    cin>>s>>t;
    if((s.size()+t.size())%2!=0){
        cout<<-1<<endl;
        return 0;
    }
    if(s==t){
        cout<<s<<endl;
        return 0;
    }
    n=s.size();
    m=t.size();
    int tot=(n+m)/2;
    for(int i=0;i<n;i++){
        if(i+1>tot)continue;
        int sz=tot-(i+1);
        if(sz>m)continue;
        string ret,ret2;
        for(int j=0;j<sz;j++)ret+=t[j];
        for(int j=0;j<=i;j++)ret+=s[j];
        for(int j=sz;j<m;j++)ret2+=t[j];
        for(int j=i+1;j<n;j++)ret2+=s[j];
        if(ret==ret2){
            cout<<ret<<endl;
            return 0;
        }
    }
    if(m>=tot){
        string ret,ret2;
        for(int j=0;j<tot;j++){
            ret+=t[j];
        }
        for(int j=tot;j<m;j++){
            ret2+=t[j];
        }
        for(int i=0;i<n;i++)ret2+=s[i];
        if(ret==ret2){cout<<ret<<endl;return 0;}
    }
    cout<<-1<<endl;

    return 0;
}