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
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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.InputStreamReader;
import java.io.IOException;
import java.util.InputMismatchException;
import java.util.Set;
import java.io.BufferedReader;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.util.StringTokenizer;
import java.util.HashSet;
import java.io.InputStream;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 * @author Zakhar Voit
 */
public class Main {
	public static void main(String[] args) {
		InputStream inputStream = System.in;
		OutputStream outputStream = System.out;
		Scanner in = new Scanner(inputStream);
		PrintWriter out = new PrintWriter(outputStream);
		TaskB solver = new TaskB();
		solver.solve(1, in, out);
		out.close();
	}
}

class TaskB {
    public void solve(int testNumber, Scanner in, PrintWriter out) {
        int n = in.nextInt();
        String[] s = new String[n];

        for (int i = 0; i < n; i++) {
            s[i] = in.nextToken();
            for (int j = 0; j < s[i].length(); j++) {
                for (int k = j; k < s[i].length(); k++) {
                    all.add(s[i].substring(j, k + 1));
                }
            }
        }

        for (int len = 1; len <= 20; len++) {
            if (check(len, "")) {
                out.println(ans);
                return;
            }
        }
    }

    String ans = null;
    Set<String> all = new HashSet<String>();

    boolean check(int len, String s) {
        if (len == 0) {
            if (!all.contains(s)) {
                ans = s;
                return true;
            } else
                return false;
        }

        for (char c = 'a'; c <= 'z'; c++) {
            if (check(len - 1, s + c))
                return true;
        }

        return false;
    }

}

class Scanner {
    BufferedReader in;
    StringTokenizer tok;

    public Scanner(InputStream in) {
        this.in = new BufferedReader(new InputStreamReader(in));
        tok = new StringTokenizer("");
    }

    public String nextToken() {
        if (!tok.hasMoreTokens()) {
            try {
                String newLine = in.readLine();
                if (newLine == null)
                    throw new InputMismatchException();
                tok = new StringTokenizer(newLine);
            } catch (IOException e) {
                throw new InputMismatchException();
            }
            return nextToken();
        }

        return tok.nextToken();
    }

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

    }

