from collections import defaultdict
import random


def programming_team_selection():
    while True:
        n_pairs = int(input())
        if n_pairs == 0:
            return
        adj_list = defaultdict(lambda: [])
        for i in range(n_pairs):
            s1, s2 = input().split()
            adj_list[pair[0]].append(s1)
            adj_list[pair[1]].append(s2)
        if clique_thing(adj_list):
            print("possible")
        else:
            print("impossible")


def find_teamless(has_team):
    keys = list(has_team.keys())
    random.shuffle(keys)
    for key in keys:
        if has_team[key] == False:
            return key
    return None


def clique_thing(adj_list):
    #def print(x):
    #    pass
    #print("====== New Test Case ======")
    #print(adj_list)
    def local_recurrer(used, teams):
        #print("Recursion with:")
        #print(used)
        #print(teams)
        # find first unused guy, he's the team captain ;)
        captain = find_teamless(used)
        # if there isn't one, everyone has a clique and we're happy
        if captain is None:
            return True
        used[captain] = True
        #print(f"The captain is {captain}")

        # otherwise we have to fill the team
        for lieutenant in adj_list[captain]:
            if used[lieutenant]:
                continue
            used[lieutenant] = True
            # ok, we have a possible second guy
            for third_guy in adj_list[captain]:
                if used[third_guy] or third_guy not in adj_list[lieutenant]:
                    continue
                used[third_guy] = True
                # and a possible third guy
                teams.append([captain, lieutenant, third_guy])
                if local_recurrer(used, teams):
                    return True
                used[third_guy] = False
                teams.remove([captain, lieutenant, third_guy])
            used[lieutenant] = False
        #print("Got stuck!")
        used[captain] = False
        return False

    teams = []
    used = {name: False for name in adj_list.keys()}
    result = local_recurrer(used, teams)
    if result:
        print(teams)
        return teams
    else:
        return None


def main():
    programming_team_selection()


if __name__ == '__main__':
    main()