main.py

import itertools;
numberOfLines = int(input())

statements = []
for i in range(0, numberOfLines):
    statements.append(input())

letters = []

for statement in statements:
    letter = statement.replace("and", "").replace("not", "").replace(" ", "")
    if letter[0] not in letters:
        letters.append(letter[0])
    if letter[1] not in letters:
        letters.append(letter[1])

table = list(itertools.product([False, True], repeat=len(letters)))

output = ""
for truthCombination in table:
    if True not in truthCombination:
        continue
    hitCombination = True
    declarations = {}
    for i in range(len(letters)):
        declarations[letters[i]] = truthCombination[i]
    for statement in statements:

        parts = statement.split(" and ")
        result = False
        if not parts[1].startswith("not"):
            result = declarations[parts[0]] and declarations[parts[1]]
        else:
            if parts[0].startswith('not'):
                result = not declarations[parts[0].replace("not ", "")] and not declarations[parts[1].replace("not ", "")]
            else:
                result = declarations[parts[0]] and not declarations[parts[1].replace("not ", "")]

        if result == True:
            hitCombination = False
            break
    if hitCombination:
        for i in range(len(truthCombination)):
            if truthCombination[i] == True:
                output += letters[i]
        break

if output == "":
    print("IMPOSSIBLE")
else:
    print(output)