#include <bits/stdc++.h>
 
using namespace std;
typedef long long ll;

int main(){
// #ifndef ONLINE_JUDGE
//   freopen("in.txt", "r", stdin);
// #endif
ios_base::sync_with_stdio(0);
cin.tie(0);

  string a, b;  cin  >> a >> b;
  string s1 = a, s2 = b;

  int i1 = 0, j1 = b.size() / 2;
  int i2 = a.size() / 2, j2 = 0;
  while(true){
    if(i1 == s1.size() / 2)
      break;
    if(i2 == s2.size())
      break;
    s1[i1] = b[j1]; i1++; j1++;
    s2[i2] = a[j2]; i2++; j2++;
  }
  if(s1 == s2)
    cout << s1;
  else
    cout << -1;

  

 
  return 0;
}