def oi(): return int(input())
def os(): return input().rstrip()
def mi(): return list(map(int, input().split()))

# import sys
# input = sys.stdin.readline
# import sys
# sys.setrecursionlimit(10**8)
# import pypyjit
# pypyjit.set_param('max_unroll_recursion=-1')
from collections import defaultdict
input_count = 0

N = oi()
dicts = {i:defaultdict(int) for i in range(1,12)}
for _ in range(N):
    S = "".join(sorted(list(os())))
    dicts[len(S)][S] += 1
alpha = [chr(ord("a")+i) for i in range(26)]
flg = False
for i in range(1, 11):
    for k in dicts[i].keys():
        for a in alpha:
            temp = "".join(sorted(list(k+a)))
            count = 0
            for j in range(len(temp)):
                temp2 = temp[:j] + temp[j+1:]
                if temp2 in dicts[i]:
                    count += dicts[i][temp2]
            if count == 1:
                flg = True
                break
    if flg:
        break
print(k+a)