Carlo’s Library

Carlo loves organizing his library and has arranged all books in lexicographical order. To enhance their appearance, he wants to insert a new book between two existing titles. The two titles, $A$ and $B$, are given as strings containing only lowercase letters of the English alphabet.

Task

Your task is to help Carlo find a title $C$, also consisting of only lowercase letters of the English alphabet for the new book such that:

• $C$ is lexicographically strictly between $A$ and $B$.
• The length of $C$ is minimized.

Help Carlo find a title $C$ satisfying these requirements, or determine that it doesn't exist.

Input data

The input file consists of:

• a line containing string $A$.
• a line containing string $B$.

Output data

The output file must contain a single line consisting of string $C$ or -1 if it doesn't exist. If there is more than one correct answer, you can print any.

Constraints and clarifications

• $1 \le \text{len}(A), \text{len}(B) \le 1 \ 000 \ 000$
• $A < B$ lexicographically.
• $A$ and $B$ contain only lowercase letters of the English alphabet.

Example 1

stdin

abc
abca

stdout

-1

Explanation

There is no title $C$ that satisfies the constraints.

Example 2

stdin

abc
def

stdout

c

Explanation

We have that abc $<$ c $<$ def, and there is no shorter title $C$ that satisfies the constraints.

Note that $C =$ b or $C =$ d are strings of length $1$ which satisfy the constraints and are, therefore, accepted by the checker.

Example 3

stdin

pcn
pk

stdout

pf

Example 4

stdin

mppxtzmo
mppxu

stdout

mppxtzz

Example 5

stdin

abc
abcaa

stdout

abca