#include <bits/stdc++.h>
using namespace std;
int main() {
  char x[101], y[101], z[101];
  cin >> x >> y;
  int n = strlen(x), i;
  for (i = 0; i < n; i++) {
    if (x[i] == y[i])
      z[i] = 'z';
    else if (x[i] > y[i])
      z[i] = y[i];
    else {
      cout << -1;
      return 0;
    }
  }
  for (i = 0; i < n; i++) cout << z[i];
  return 0;
}
