#!/usr/bin/env python

import re

RE_LINE = re.compile('^Step (.*) must be finished before step (.*) can begin.')
tree = dict()

def read_input():
    with open('input.txt', 'r') as f:
        for line in f:
            yield line

def add_dependency(a, b):
    if a not in tree:
        tree[a] = {
            'node': a,
            'parent': '',
            'child': '',
        } 
    if b not in tree:
        tree[b] = {
            'node': b,
            'parent': '',
            'child': '',
        } 
    tree[a]['child'] += b
    tree[b]['parent'] += a

for l in read_input():
    m = RE_LINE.match(l)
    if m is None:
        raise Exception('unpaseable line')
    add_dependency(m.group(1), m.group(2))

result = []
while len(tree):
   root = min([ n['node'] for n in tree.values() if len(n['parent']) == 0])
   for c in tree[root]['child']:
       tree[c]['parent'] = tree[c]['parent'].replace(root, "")
   del tree[root]
   result.append(root)


print(''.join(result))