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
w
u
q
l
y
i
o
a
u
 5
splt
ohqykk
xqpz
knojbur
pmfm
 2
hk
obsp
 5
zzxpfk
abcdefghijklmnopqrst
uvwxz
srgkjaskldfkln
vgnsdfdgfh
 2
rxscdzkkezud
wjehahqgouqvjienq
 5
ojdfhi
abcdefghijklmnopqrst
uvwxyz
ddfhdfhlasjt
qqq
 1
epkotfpkkrhhmuipmtdk
 10
kpmwcdoysw
gtpr
kuzoxmiixxbl
crgqtuo
jhbplhpklrgwnaugdf
zuxdaat
aycv
qwghrkqwkobrgevsjrk
tdxgc
lxyzgcmbzulcst
 5
dfijdfhi
abcdefghijklmnopqrst
uvwxy
kopsdfgiopjipw
jty
 2
xlaxwpjabtpwddc
txwdjmohrrszorrnomc
 1
a
 30
waiphwcqrrinr
o
qiqehzmgsjdoqd
kjexeesevrlowxhghq
judikhzkj
z
xo
lsdzypkfqro
sshgcxsky
gecntpcmoojfwp
svmytmcfhc
jrsrvsvbaiumlmkptn
s
wpcsovfjlyspviflk
ktvyzvddgllht
szahigtmklglrcocbo
znligfxkgxzkcfeu
liryvzmqwhr
xgrxkgiehxztv
etrjxdczppafly
jrdgajschhwsci
doxnxbjwptnimjmsuijx
zciwkbvrhgsjhrr
ehh
bzlnmd
xpbtbpftimnn
khbknqbvdkdd
mldj
skrvnyz
avptgpjmenfzfxwckmt
 30
e
x
itaubd
xcn
v
x
i
ci
aqs
zbi
q
cvdu
a
jos
qy
fjf
nayke
ge
s
q
qwad
gvz
o
ubn
nzoc
h
wnjwjrf
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.File;
import java.io.FileReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.HashMap;
import java.util.HashSet;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;
import java.util.PriorityQueue;
import java.util.Queue;
import java.util.Set;
import java.util.StringTokenizer;

public class Main {
    
    String s[];
    int n;
    Queue<String> togo = new LinkedList<>();
    
    private void solve() throws IOException {
        n = nextInt();
        s = new String[n];
        for (int i = 0; i < n; ++i)
            s[i] = nextToken();
        
        togo.add("");
        go();
    }
    
    private void go() {
        while (true) {
            String str = togo.poll();
            for (char ch = 'a'; ch <= 'z'; ++ch) {
                String str2 = str + ch;
                togo.add(str2);
                boolean ct = false;
                for (int i = 0; i < n; ++i)
                    if (s[i].contains(str2)) {
                        ct = true;
                        break;
                    }
                if (!ct) {
                    println(str2);
                    return;
                }
            }
        }
    }
    
    
    public static void main(String[] args) {
        new Main().run();
    }

    public void run() {
        try {
            if (isFileIO) {
                pw = new PrintWriter(new File("output.out"));
                br = new BufferedReader(new FileReader("input.in"));
            } else {
                pw = new PrintWriter(System.out);
                br = new BufferedReader(new InputStreamReader(System.in));
            }
            solve();
            pw.close();
            br.close();
        } catch (IOException e) {
            System.err.println("IO Error");
        }
    }

    private void print(Object o) {
        pw.print(o);
    }

    private void println(Object o) {
        pw.println(o);
    }

    private void println() {
        pw.println();
    }

    int[] nextIntArray(int n) throws IOException {
        int arr[] = new int[n];
        for (int i = 0; i < n; ++i)
            arr[i] = Integer.parseInt(nextToken());
        return arr;
    }

    int nextInt() throws IOException {
        return Integer.parseInt(nextToken());
    }

    long nextLong() throws IOException {
        return Long.parseLong(nextToken());
    }

    double nextDouble() throws IOException {
        return Double.parseDouble(nextToken());
    }

    private String nextToken() throws IOException {
        while (tokenizer == null || !tokenizer.hasMoreTokens()) {
            tokenizer = new StringTokenizer(br.readLine());
        }

        return tokenizer.nextToken();
    }

    private BufferedReader br;
    private StringTokenizer tokenizer;
    private PrintWriter pw;
    private final boolean isFileIO = false;
}