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.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.util.Comparator;
import java.util.StringTokenizer;
import java.util.TreeSet;

public class P_278_B {
    
static  class MyComparator implements Comparator<String> {
          public int compare(String a, String b) {
          int aa=a.length();
          int bb=b.length();
          if(aa!=bb)
              return (aa>bb)?1:-1;

            return a.compareTo(b);
          }};
    static TreeSet<String> S = new TreeSet<String>(new MyComparator());

    public static void main(String[] args) {

        InputReader in = new InputReader(System.in);
        int n = in.nextInt();
        String[] arr = new String[n];
        for (int i = 0; i < n; i++)
            arr[i] = in.nextLine().trim();

        int c = 2;

        check(c, new StringBuilder());
        
        
        for (int i = 0; i < n; i++) {
            int z = arr[i].length();
            for (int j = 0; j < z; j++)
                check2(c, new StringBuilder(), arr[i], j, arr[i].length());
        }
        System.out.println(S.pollFirst());
    }

    private static void check2(int c, StringBuilder s, String arr, int j, int n) {
        if (c == -1 || j == n)
            return;
        StringBuilder k = new StringBuilder(s).append(arr.charAt(j));
        //System.out.println(k.toString());
        S.remove(k.toString());
        check2(c - 1, k, arr, j + 1, n);
    }

    private static void check(int c, StringBuilder s) {
        if (c == -1)
            return;
        char k;
        StringBuilder t;
        for (int i = 0; i < 26; i++) {
            k = (char) ('a' + i);
            t = new StringBuilder(s).append(k);
            //System.out.println(t.toString());
            S.add(t.toString());
            check(c - 1, t);
        }
    }

    static class InputReader {
        private BufferedReader reader;
        private StringTokenizer tokenizer;

        public InputReader(InputStream stream) {
            reader = new BufferedReader(new InputStreamReader(stream));
            tokenizer = null;
        }

        public String nextLine() {
            try {
                return reader.readLine();
            } catch (IOException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
                return null;
            }
        }

        public String next() {
            while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        }

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

        public long nextLong() {
            return Long.parseLong(next());
        }
    }

}
