\problemname{Keyboardd}
\illustration{.4}{keyboard}{\href{http://www.thekeyboardwaffleiron.com/}{Waffle keyboard} by The Keyboard Waffle Iron}%
Oh no! Joanna just spilled some syrup on her keyboard and now some of the keys
are sticky. This causes her considerable frustration, as every time she presses
one of the sticky keys, the corresponding character gets entered twice on her
computer.

This could not have happened at a more inconvenient time; it is the start of
the contest and she was just about to type in the solution to the first
problem! Joanna does not have time to remove and clean every key on her
keyboard, so she wonders if there is a way to quickly identify the sticky keys.
Starting to panic, she quickly types some text on her keyboard and stares at
the resulting text displayed on her screen.

Given the line of text that Joanna typed on her keyboard and the resulting text
displayed on her screen, help her determine which keys must be sticky.

\begin{Input}
The input consists of:
\begin{itemize}
    \item One line containing a string $s$ ($1 \leq \mathrm{length}(s) \leq 1\,000$), the text that Joanna typed on her keyboard.
    \item One line containing a string $t$ ($1 \leq \mathrm{length}(t) \leq 1\,000$), the text displayed on Joanna's screen as a result.
\end{itemize}

Both $s$ and $t$ consist only of lower-case letters (`\texttt{a}'--`\texttt{z}') and
spaces (`\texttt{ }'), and start and end with a letter.

It is guaranteed that $t$ is the result of doubling each
character in $s$ that corresponds to a sticky key. At least one character in
$s$ corresponds to a sticky key (i.e.\ $s \neq t$).
\end{Input}

\begin{Output}
Output all characters (letters and space) corresponding to keys that must be
sticky, in any order.
\end{Output}
