#include <bits/stdc++.h>
int main() {
  char xn[110] = {0}, yn[110] = {0}, zn[110] = {0};
  int flag, i;
  flag = 0;
  gets(xn);
  gets(yn);
  for (i = 0; xn[i] != '\0'; i++) {
    if (xn[i] < yn[i]) {
      flag = 1;
      break;
    } else if (xn[i] == yn[i]) {
      zn[i] = 'z';
    } else {
      zn[i] = yn[i];
    }
  }
  if (flag)
    printf("-1");
  else {
    printf("%s", zn);
  }
  return 0;
}
