#!/usr/bin/env python3

from functools import reduce
from pprint import pprint

with open('input.txt', 'r') as f:
    instructions = [x.strip() for x in f.readlines()]
    instructions = [(x.split(' ')[1], x.split(' ')[7]) for x in instructions]

steps = {y for x in instructions for y in x}  # flatten
finished = list()
requirements = { step: [] for step in steps }
for el in instructions:
    requirements[el[1]].append(el[0])


print(requirements)
print(steps)

while len(steps) != 0:
    possible = [ step for step in requirements if len(requirements[step]) == 0]
    possible.sort()
    step = possible[0]
    steps.remove(step)
    requirements.pop(step)
    for el in requirements:
        if step in requirements[el]:
            requirements[el].remove(step)
    finished.append(step)
    
print(''.join(finished))