common.py

import sys

def read(): return raw_input()

def selected_employees():
    return (read(), read())

def read_relation():
    manager, managed = read().split()
    return managed, manager

def read_relations(count):
    d = {}
    while True:
        try:
            m,s = read_relation()
            if not d.has_key(s):
                d[s] = None
            d[m] = s
        except:
            return d

def cons(a, l):
    aux = [a]
    aux.extend(l)
    return aux

def walk_managers(a, relations):
    if a is None:
        return []
    else:
        return cons(a, walk_managers(relations[a], relations))

def rev(l):
    l.reverse()
    return l

def last(l):
    return l[-1]

def common_manager(a,b,relations):
    managers_a = walk_managers(a, relations)
    managers_b = walk_managers(b, relations)
    top_to_a = rev(managers_a)
    top_to_b = rev(managers_b)
    commons = zip(top_to_a, top_to_b)
    return last([a for a,b in zip(top_to_a, top_to_b) if a == b])

def process_common_manager(count):
    s1, s2 = selected_employees()
    relations = read_relations(count)
    return common_manager(s1, s2, relations)

def OutputCommonManager(count):
    common = process_common_manager(count)
    sys.stdout.write(common)

_count = int(raw_input())
OutputCommonManager(_count)