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'''
Created on 28.02.2013

@author: Sergei Zalivako
'''

import sys

def get_code(s):
    sum = 0
    for si in s:
        sum = sum * 26 + ord(si) - ord('a') + 1
    return sum

def get_s(c):
    s = ''
    while (c > 0):
        s = chr(ord('a') + c % 26 - 1) + s
        c /= 26
    return s

n = int(raw_input())
s = []
for i in range(n):
    si = raw_input()
    s.append(si)

sub = [[] for i in range(20)]

for i in range(n):
    for si in s:
        for l in range(1, len(si) + 1):
            for j in range(len(si)):
                if (j + l <= len(si)):
                    sub[l-1].append(si[j:j+l])            
for i in range(20):
    sub[i].sort()
st = 0
for ss in sub:
    st += 1
    prev_code = get_code(ss[0])
    for si in ss:
        cur_code = get_code(si)
        #print si, cur_code
        if (cur_code != prev_code):
            if (cur_code - prev_code > 1):
                print get_s(prev_code + 1)
                sys.exit(0)
            else:
                prev_code = cur_code
    if (cur_code != 26 ** st):
        print get_s(cur_code + 1)
        sys.exit(0)
            

