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import sys
import itertools
import string
read_ints = lambda : map(int, datum.readline().split())
read_strs = lambda : map(str, datum.readline().split())

def dbg(was):
    if debug_mode:
        print 'DBG\t' + str(was)

def check(strr, data):
    for z in data:
        if strr in z:
            return False
    return True

def solve(n, data):
    symbs = 1
    letters = string.ascii_letters
    letters = letters[:len(letters)/2]
    while True:
        for x in itertools.combinations_with_replacement(letters, symbs):
            x = ''.join(x)
            if check(x, data):
                return x
        symbs += 1
    

if len(sys.argv) > 1:
    debug_mode = True
    datum = open(sys.argv[1], 'r')
else:
    debug_mode = False
    datum = sys.stdin

n = read_ints()[0]
data = []
for _ in xrange(n):
    data.append(read_strs()[0])
    
#dbg((n, n**2))
#dbg(data)
print str(solve(n, data))


