3
rjnflsbpxqivrcdjptj
vpojopbwbwbswdu
rydkiwnugwddcgcrng
 1
z
 30
b
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f
m
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k
p
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a
c
w
z
z
p
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d
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 5
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 2
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 5
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abcdefghijklmnopqrst
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vgnsdfdgfh
 2
rxscdzkkezud
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 5
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abcdefghijklmnopqrst
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qqq
 1
epkotfpkkrhhmuipmtdk
 10
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jhbplhpklrgwnaugdf
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 5
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abcdefghijklmnopqrst
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 2
xlaxwpjabtpwddc
txwdjmohrrszorrnomc
 1
a
 30
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xpbtbpftimnn
khbknqbvdkdd
mldj
skrvnyz
avptgpjmenfzfxwckmt
 30
e
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 1
b

a
 a
 e
 a
 a
 y
 b
 aa
 a
 ab
 z
 e
 b
 aa
 l
 a

from Queue import*
PROBLEM = 2
FILE = "P%i" %(PROBLEM) 
outFile = None
#outFile = "P%i_output" %(PROBLEM)
try:
    inFile = open(FILE+".txt")
except:
    pass
if outFile is not None:
    outFile = open(outFile+".txt","w")

def read():
    try:
        return inFile.readline().strip()
    except:
        return raw_input().strip()

def write(*args):
    newline = True
    if type(args[-1]) == bool:
        newline = args[-1]
        args = args[:-1]
    
    output = ""
    for i in xrange(len(args)):
        if i > 0:
            output += " "
        output += str(args[i])
        
    if outFile is None:
        if newline:
            print output
        else:
            print output,
            
    else:
        if newline:
            outFile.write(output+"\n")
        else:
            outFile.write(output+" ")
   

            
n = int(read())
strings = ""
for _ in xrange(n):
    strings += read()+" "
    
q = Queue()
q.put("")

while not q.empty():
    s = q.get()
    if s not in strings:
        ans = s
        break
    
    for char in xrange(ord('a'),ord('z')+1):
        q.put(s+chr(char))

write(ans)      
        
if outFile is not None:
    outFile.close()
