#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define pb push_back
#define f first
#define s second
#define all(v) (v).begin(),(v).end()
#define rall(v) (v).rbegin(),(v).rend()

const int inf = 1e9;
const ll infll = 1e18;
const int M = 1e9 + 7;
const int N = 4e5;

int main(){
	string s, t;
	cin >> s >> t;
	int n = s.size(), m = t.size();
	if((n+m)%2){
		puts("-1");
		return 0;
	}
	string ss[2] = {"",""}, tt[2] = {"",""};
	int midn = (n+m)/2;
	for (int i = 0; i < n; ++i){
		int prefLen = i+1;
		int lenNow = n - prefLen;
		int suffLen = midn - lenNow;

		if(suffLen <= 0) continue;


		ss[0] = s.substr(0, prefLen);
		ss[1] = s.substr(prefLen);

		tt[0] = t.substr(0, n-suffLen);
		tt[1] = t.substr(n-suffLen);
		if((tt[1] + ss[1]) == (tt[0] + ss[0])){
			cout << (tt[1] + ss[1]) << endl;
			return 0;
		}
	}
	puts("-1");
}