#include <bits/stdc++.h>
using namespace std;
#define ll long long int
#define ld long double
#define pb push_back
#define mp make_pair
#define F first
#define S second
#define PI 3.14159265
#define fast ios::sync_with_stdio(false),cin.tie(NULL),cout.tie(NULL)
typedef pair<ll,ll> pli;
typedef pair<int,int> pii;
typedef pair<ld,ld> pld;
ll MOD=1000000007 ;
ll MX=1000000000000000;

char a[5050];
char b[5050];
int main()
{
    fast;
    //freopen("input.txt","r",stdin);
    cin>>(a+1);
    cin>>(b+1);
    ll n=strlen(a+1);
    ll m=strlen(b+1);
    if((n+m)%2==1)
    {
        cout<<"-1\n";
    }
    else
    {
        string res="";
        string pref="";
        string suff="";
        ll stx=-1,sty=-1;
        if(n<=m)
        {
            stx=0;
            sty=(m-n)/2;
        }
        else
        {
            stx=(n-m)/2;
            sty=0;
        }
        for(int i=0;i<=n;i++)
        {
            ll x=i+stx;
            ll y=sty+i;
            if(x!=0)
                pref+=a[x];
            if(y>m)
                continue;
            if(y!=0)
            {
                suff=b[m-y+1]+suff;
            }
            string a1=suff;
            for(int j=x+1;j<=n;j++)
                a1+=a[j];
            string b1="";
            for(int j=1;j<=m-y;j++)
                b1+=b[j];
            b1+=pref;
            ll temp=1;
            for(int j=0;j<a1.size();j++)
            {
                if(a1[j]!=b1[j])
                    temp=0;
            }
            if(temp)
            {
                res=a1;
                break;
            }
        }
        if(res=="")
        {
            cout<<"-1\n";
        }
        else
        {
            cout<<res<<endl;
        }

    }

    return 0;
}