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 2
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 2
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import string
digs = string.digits + string.ascii_letters
from bisect import *
gamma=[]
def int2base(x, base):
    if x < 0:
        sign = -1
    elif x == 0:
        return digs[0]
    else:
        sign = 1

    x *= sign
    digits = []

    while x:
        digits.append(digs[int(x % base)])
        x = int(x / base)

    if sign < 0:
        digits.append('-')

    digits.reverse()

    return ''.join(digits)


a=int(input())
ans=[]
for i in range(a):
    s=input()
   
    for i in range(len(s)):
        for j in range(i,len(s)):
            t=s[i:j+1]
            
            total=0
            
            if(len(t)>4):
                continue;
            else:
                for m in range(len(t)):
                    total+=pow(26,len(t)-m-1)*(ord(t[m])-ord('a')+1)
                
            gamma.append(total)
            
            
gamma=list(set(gamma))
gamma.sort()

if(1 not in gamma):
    print('a')
    exit()


else:
    for j in range(1,len(gamma)):
        if(gamma[j]!=j+1):
            val=gamma[j-1]+1
            timp=''
            while(val):
                rem=val%26
                timp+=chr(97+rem-1)
                val=val//26
            print(timp[::-1])
            exit()

val=gamma[-1]+1
timp=''
while(val):
    rem=val%26
    timp+=chr(97+rem-1)
    val=val//26
    

print(timp[::-1])




            
           
