#include <bits/stdc++.h>
#define quick ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL);
#define F first
#define S second
#define pb push_back
#define mp make_pair
#define ll long long
#define ull unsigned ll
#define pii pair <int,int>
#define pll pair <ll,ll>
#define INF 1000000000
#define mod 1000000007
#define size(a) ((int)(a.size()))
using namespace std;
void solv()
{
	string s,ss;
	cin>>s>>ss;
	if(s==ss)
    {
        cout<<s<<endl;
        return;
    }
    if(size(s)!=size(ss) || size(s)%2)
    {
        cout<<-1<<endl;
        return;
    }
    for(int i=0,j=size(s)/2;j<size(s);i++,j++)
    {
        if(s[i]!=s[j])
        {
            cout<<-1<<endl;
            return;
        }
    }
    for(int i=0,j=size(ss)/2;j<size(ss);i++,j++)
    {
        if(ss[i]!=ss[j])
        {
            cout<<-1<<endl;
            return;
        }
    }
    for(int i=size(s)/2;i<size(s);i++) cout<<ss[i];
    for(int i=size(s)/2;i<size(s);i++) cout<<s[i];
    cout<<endl;
}
int main()
{
	quick
	int TT=1; //   cin>>TT;
	while(TT--)  solv();

	return 0;
}