3
rjnflsbpxqivrcdjptj
vpojopbwbwbswdu
rydkiwnugwddcgcrng
 1
z
 30
b
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n
f
m
t
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j
k
p
h
a
c
w
z
z
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d
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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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kuzoxmiixxbl
crgqtuo
jhbplhpklrgwnaugdf
zuxdaat
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qwghrkqwkobrgevsjrk
tdxgc
lxyzgcmbzulcst
 5
dfijdfhi
abcdefghijklmnopqrst
uvwxy
kopsdfgiopjipw
jty
 2
xlaxwpjabtpwddc
txwdjmohrrszorrnomc
 1
a
 30
waiphwcqrrinr
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qiqehzmgsjdoqd
kjexeesevrlowxhghq
judikhzkj
z
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jrsrvsvbaiumlmkptn
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ktvyzvddgllht
szahigtmklglrcocbo
znligfxkgxzkcfeu
liryvzmqwhr
xgrxkgiehxztv
etrjxdczppafly
jrdgajschhwsci
doxnxbjwptnimjmsuijx
zciwkbvrhgsjhrr
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bzlnmd
xpbtbpftimnn
khbknqbvdkdd
mldj
skrvnyz
avptgpjmenfzfxwckmt
 30
e
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xcn
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x
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qy
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nayke
ge
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nzoc
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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.*;
import java.util.HashSet;
import java.util.Scanner;
import java.util.Set;

/**
 * User: Oleksiy Pylypenko
 * Date: 1/6/13
 * Time: 1:46 PM
 */
public class B {
    static boolean debug = !"true".equalsIgnoreCase(System.getProperty("ONLINE_JUDGE"));
    private Scanner in;
    private PrintWriter out;

    private void solve() throws IOException {
        int n = in.nextInt();
        Set<String> hash = new HashSet<String>(n * n * n);
        for (int i = 0; i < n; i++) {
            String str = in.next();
            for (int j = 0; j < str.length(); j++) {
                for (int k = j + 1; k <= str.length(); k++) {
                    hash.add(str.substring(j, k));
                }
            }
        }
        StringBuilder builder = new StringBuilder("a");
        for (int i = 1; ; i++) {
            String str = builder.toString();
            if (!hash.contains(str)) {
                out.println(str);
                return;
            }
            boolean carry = true;
            for (int j = builder.length() - 1; carry && j >= 0; j--) {
                char c = builder.charAt(j);
                if (carry) {
                    if (c < 'z') {
                        c++;
                        carry = false;
                    } else {
                        c = 'a';
                    }
                }
                builder.setCharAt(j, c);
            }
            if (carry) {
                builder.insert(0, 'a');
            }
        }
    }

    public static void main(String[] args) throws IOException {
        new B().exec();
    }

    private void exec() {
        if (debug) {
            try {
                System.setIn(new FileInputStream(getClass().getName() + ".in"));
            } catch (FileNotFoundException e) {
                System.out.println(e);
                System.exit(1);
            }
        }
        in = new Scanner(new InputStreamReader(System.in));
        out = new PrintWriter(System.out, true);

        try {
            if (debug) {
                while (in.hasNext()) {
                    solve();
                }
            } else {
                solve();
            }
        } catch (IOException e) {
            if (debug) {
                System.out.println(e);
                System.exit(1);
            }
        } finally {
            out.close();
            in.close();
        }
    }

}
