P14896 [ICPC 2018 Yokohama R] Shortest Common Non-Subsequence

A subsequence of a sequence $P$ is a sequence that can be derived from the original sequence $P$ by picking up some or no elements of $P$ preserving the order. For example, "ICPC" is a subsequence of "MICROPROCESSOR". A common subsequence of two sequences is a subsequence of both sequences. The famous longest common subsequence problem is finding the longest of common subsequences of two given sequences. In this problem, conversely, we consider the shortest common non-subsequence problem: Given two sequences consisting of 0 and 1, your task is to find the shortest sequence also consisting of 0 and 1 that is a subsequence of neither of the two sequences.

The input consists of a single test case with two lines. Both lines are sequences consisting only of 0 and 1. Their lengths are between 1 and 4000, inclusive.

Output in one line the shortest common non-subsequence of two given sequences. If there are two or more such sequences, you should output the lexicographically smallest one. Here, a sequence $P$ is lexicographically smaller than another sequence $Q$ of the same length if there exists $k$ such that $P_1 = Q_1$, $\ldots$, $P_{k-1} = Q_{k-1}$, and $P_k < Q_k$, where $S_i$ is the $i$-th character of a sequence $S$.