#include <bits/stdc++.h>

#define ll long long
#define pb push_back
#define x first
#define y second
#define sz(u) (int)(u.size())
#define ld long double
#define all(u) u.begin(),u.end()

using namespace std;


int main()
{
    ios::sync_with_stdio(0); cin.tie(0);
    string a,b; cin>>a>>b;
    int c=a.length(), d=b.length();
    if(c%2!=d%2){
        cout<<-1<<endl;
        return 0;
    }
    int l = (c+d)/2;
    for(int i=0;i<=d;i++){
        int need=l-i;
        if(need>c) continue;
        string tmp1="",tmp2="";
        for(int k=0;k<i;k++) tmp2+=b[k];
        for(int k=0;k<need;k++) tmp2+=a[k];
        for(int k=i;k<d;k++) tmp1+=b[k];
        for(int k=need;k<c;k++) tmp1+=a[k];
        if(tmp1==tmp2){
            cout<<tmp1<<endl;
            return 0;
        }
    }
    cout<<-1<<endl;
    return 0;
}