/*
	Failure is not the reason for you to stop trying
	   it is actually a remainder that you should
			KEEP GOING
*/
#include<bits/stdc++.h>
#define ll long long
#define SaveTime ios_base::sync_with_stdio(false), cin.tie(0);
using namespace std;

int main()
{
   SaveTime
   string a,b; cin >> a >> b;
   if (a == b) {
      cout << a;
      return 0;
   }
   else {
      int n1 = a.size(), n2 = b.size();
      if ((n1+n2) %2) {
         cout << -1;
         return 0;
      }
      string ans = "-1";
      for (int i = 0; i < n1; i++) {
         string s1 = a.substr(0, i+1);
         int nb = (n1+n2)/2;
         int rem = n2+i+1-nb;
         if (rem < 0)
            continue;
         if (n2-rem < 0) {
            continue;
         }
         string s2 = b.substr(n2-rem, rem);
         string ch1 = s2+a.substr(i+1, n1-i-1);
         string ch2 = b.substr(0, n2-rem) + s1;
         //cout << ch1 << ' ' << ch2 << endl;
         if (ch1 == ch2) {
            ans = ch1;
            break;
         }
      }
      cout << ans;
   }
}