\problemname{Flipping Cards}

\illustration{0.3}{card.png}

%
% https://openclipart.org/detail/168415/playing-card
% public domain 

Mike and his young daughter Jesse are playing a new card game meant for kids.
The rules are quite simple, each player is dealt a hand of cards.
Each card has one picture on each side. They take turns playing cards
and the first one to run out of cards is the winner.

A player's turn consists of picking a subset of cards from their hand
and laying them on the table. The only rule is that the cards must be
placed on the table such that no two cards are showing the same picture.

Mike thought this was a very appropriate game to play with his kid because
of the simple rules. Mike also liked this game because finding the best
strategy is an algorithmically interesting challenge!

Help Mike determine if he can play his entire hand on his first round.

\section*{Input}

The first line of the input contains a single positive integer $T$ ($T \leq 10$) indicating
the number of test cases.
%\footnote{Howard: I put the limit here based on the provided data.  
%Please let me know if you want to change this.} 
Each test case begins with a single integer
$n$ denoting the number of cards in Mike's hand. Here $1 \leq n \leq 50\,000$.
Following this are $n$ lines, each describing a card in Mike's hand.

The pictures on the cards are represented by integers.
The $i$th card is given by two integers $p_i, q_i$ where
$1 \leq p_i, q_i \leq 2n$.

\section*{Output}

For each test case you should output a single line
with the word \texttt{possible} if it is possible for Mike to play
his entire hand in one turn, or \texttt{impossible} if Mike cannot play
his entire hand in one turn.

