3
rjnflsbpxqivrcdjptj
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 1
z
 30
b
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p
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f
m
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j
k
p
h
a
c
w
z
z
p
t
d
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w
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a
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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
wjehahqgouqvjienq
 5
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abcdefghijklmnopqrst
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ddfhdfhlasjt
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 1
epkotfpkkrhhmuipmtdk
 10
kpmwcdoysw
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jhbplhpklrgwnaugdf
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 5
dfijdfhi
abcdefghijklmnopqrst
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 2
xlaxwpjabtpwddc
txwdjmohrrszorrnomc
 1
a
 30
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kjexeesevrlowxhghq
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znligfxkgxzkcfeu
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doxnxbjwptnimjmsuijx
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ehh
bzlnmd
xpbtbpftimnn
khbknqbvdkdd
mldj
skrvnyz
avptgpjmenfzfxwckmt
 30
e
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itaubd
xcn
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x
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ci
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jos
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ge
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t
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bw
 1
b

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


import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Locale;
public class array2 {
    public static void main(String[] args) {
InputStreamReader in=new InputStreamReader(System.in);
BufferedReader con=new BufferedReader(in);
try {
int s1=Integer.parseInt(con.readLine());
String s[]=new String[s1];
for(int i=0;i<s1;i++)
    s[i]=con.readLine();
ArrayList<Character> a=new ArrayList<>();
char b='a';a.add('a');
for(int i=0;i<25;i++){
    b+=1;a.add(b);
}
for(int i=0;i<s1;i++){
    for(int j=0;j<s[i].length();j++){
        if(a.indexOf(s[i].charAt(j))!=-1){
            a.remove(a.indexOf(s[i].charAt(j)));
        }
    }
}
if(a.size()!=0)
System.out.println(a.get(0));
else{
    String w="a";int c=0;
    while(c==0){
    for(int i=0;i<s1;i++){
        if(s[i].indexOf(w)!=-1){
        c=0;    break;
        }else
            c=1;
    }
    char r=w.charAt(w.length()-1);
    w=w.concat(String.valueOf(++r));System.out.println(w);
    }
    System.out.println(w.substring(0, w.length()-1));
}
} catch (Exception e) {
    e.printStackTrace();
}
    }
}