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'''
Created on 28.02.2013

@author: Sergei Zalivako
'''

import sys

def cmp(s1, s2):
    i = len(s1) - 1
    count = 0
    while (i >= 0):
        if (ord(s1[i]) - ord(s2[i]) == 1):
            count += 1
        if (ord(s1[i]) - ord(s2[i]) > 1):
            return False
        i -= 1
    if (count != 1):
        return False
    else:
        return True

def next(s):
    i = len(s) - 1
    res = ''
    while (i >= 0 and s[i] == 'z'):
        res = 'a' + res
        i -= 1
    if (i >= 0):
        res = chr(ord(s[i]) + 1) + res
        i -= 1
    while (i >= 0):
        res = s[i] + res
        i -= 1
    return res
    
    
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
    if (ss == []):
        print 'a' * st
        sys.exit(0)
    prev_code = ss[0]
    for si in ss:
        cur_code = si
        if (cur_code != prev_code):
            if (cmp(cur_code, prev_code) == False):
                print next(prev_code)
                sys.exit(0)
            else:
                prev_code = cur_code
    if (cur_code != 'z' * st):
        print next(cur_code)
        sys.exit(0)
    
