from collections import defaultdict

n = int(input())
counter = defaultdict(set)
for _ in range(n):
    s = input()
    a_cnt, b_cnt = 0, 0
    for i in range(n):
        if s[i] == 'a':
            a_cnt += 1
        else:
            b_cnt += 1
    counter[(a_cnt, b_cnt)].add(s)
for i in range(n + 1):
    if len(counter[(i, n - i)]) == 0:
        print('a' * i + 'b' * (n - i))
        break