#include <bits/stdc++.h>
using namespace std;
int mypowerfun(int sets, int b) {
  if (b == 0) return 1;
  if (b == 1) return sets;
  int x = mypowerfun(sets, b / 2);
  if (b % 2 == 0)
    return x * x;
  else
    return x * x * sets;
}
void pri_arr(int sets[], int n) {
  int i;
  for (i = 0; i < n; i++) printf("%d ", sets[i]);
  printf("\n");
}
void pri_vec(vector<int> a, int n) {
  for (std::vector<int>::iterator it = a.begin(); it != a.end(); ++it)
    std::cout << *it;
  std::cout << '\n';
}
int mygcd(int sets, int b) {
  if (b != 0)
    return mygcd(b, sets % b);
  else
    return sets;
}
int main() {
  char a[101], b[101], c[101];
  int i;
  cin >> a >> b;
  for (i = 0; a[i] != '\0'; i++) {
    if (a[i] == b[i]) {
      c[i] = 'z';
    } else if (a[i] > b[i]) {
      c[i] = b[i];
    } else {
      printf("-1\n");
      return 0;
    }
  }
  for (i = 0; i < strlen(a); i++) {
    cout << c[i];
  }
  return 0;
}
