3
rjnflsbpxqivrcdjptj
vpojopbwbwbswdu
rydkiwnugwddcgcrng
 1
z
 30
b
u
p
n
f
m
t
i
j
k
p
h
a
c
w
z
z
p
t
d
o
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q
l
y
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 5
splt
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 2
hk
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 5
zzxpfk
abcdefghijklmnopqrst
uvwxz
srgkjaskldfkln
vgnsdfdgfh
 2
rxscdzkkezud
wjehahqgouqvjienq
 5
ojdfhi
abcdefghijklmnopqrst
uvwxyz
ddfhdfhlasjt
qqq
 1
epkotfpkkrhhmuipmtdk
 10
kpmwcdoysw
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crgqtuo
jhbplhpklrgwnaugdf
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qwghrkqwkobrgevsjrk
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lxyzgcmbzulcst
 5
dfijdfhi
abcdefghijklmnopqrst
uvwxy
kopsdfgiopjipw
jty
 2
xlaxwpjabtpwddc
txwdjmohrrszorrnomc
 1
a
 30
waiphwcqrrinr
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kjexeesevrlowxhghq
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znligfxkgxzkcfeu
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etrjxdczppafly
jrdgajschhwsci
doxnxbjwptnimjmsuijx
zciwkbvrhgsjhrr
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bzlnmd
xpbtbpftimnn
khbknqbvdkdd
mldj
skrvnyz
avptgpjmenfzfxwckmt
 30
e
x
itaubd
xcn
v
x
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ci
aqs
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jos
qy
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nayke
ge
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t
bq
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.IOException;
import java.io.InputStreamReader;
import java.util.Arrays;


public class Main{

    static int inc(char[] res){
        int p = 0;
        while(res[p] != '?')p++;
        p--;
        if(p < 0){
            p++;
            res[p] = 'a';
            return 1;
        }
        
        int tmp = p;
        while(tmp >= 0 && res[tmp] == 'z'){
            res[tmp--] = 'a';
        }
        if(tmp < 0){
            res[p+1] = 'a';
            return p+2;
        }else{
            res[tmp] = (char)(res[tmp]+1);
            return p+1;
        }
    }
    
    public static void main(String[] args) throws IOException{
        BufferedReader r = new BufferedReader(new InputStreamReader(System.in));
        int n = Integer.parseInt(r.readLine());
        char[][] cads = new char[n][];
        for(int i = 0; i < n; i++){
            cads[i] = r.readLine().toCharArray();
        }
        char[] res = new char[23];
        Arrays.fill(res, '?');
        boolean ok = false;
        int length = 0;
        while(!ok){
            length = inc(res);
            ok = true;
            for(int i = 0; i < n; i++){
                for(int j = 0; j <= cads[i].length - length; j++){
                    boolean ok1 = false;
                    for(int k = 0; k < length; k++){
                        if(cads[i][j+k] != res[k]){
                            ok1 = true;
                            break;
                        }
                    }
                    if(!ok1){
                        ok = false;
                        break;
                    }
                }
                if(!ok){
                    break;
                }
            }
        }
        System.out.println(new String(res).substring(0, length));
    }

}
