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.FileNotFoundException;
import java.io.FileReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.util.StringTokenizer;

public class B {
    static FastScanner in = new FastScanner(System.in);
    static String a[];
    public static void main (String args[]){
        int n = in.nextInt();
        a = new String[n];
        for(int i = 0 ; i < a.length;i++){
            a[i] =in.next();
        }
        String ss = "0";
        String buff[][] = new String[1000000][26];
        int c1 = 0;
        int c2 = 0;
        while(ss.equals("0")){
            String tmp = "";
            ss = solve(tmp, n, buff[c1++]);
            c2++;
            if(!ss.equals("0")){
                break;
            }
            while(ss.equals("0")){
                for(int i = 0 ; i < 26 ;i++){
                    ss = solve(buff[c1-1][i], n, buff[c2++]);
                    if(!ss.equals("0")){
                        break;
                    }
                }
                c1++;
            }
        }
        System.out.println(ss);
    }
    static String solve(String s1,int n,String buff[]){
        boolean f1 = false;
        int k = 0 ;
        for(char j = 'a';j<='z';j++){
            buff[k++]=(s1+"")+j;
            for(int i = 0 ; i < n ;i++){
                if(a[i].contains((s1+"")+j)){
                    f1 = true;
                    break;
                }
                
            }
                if(!f1){
                    return s1+j;
                }
                f1 =false;
            
        }
        return "0";
    }
}

class FastScanner {
    BufferedReader br;
    StringTokenizer st;
    public FastScanner(File f) {
        try {
            br = new BufferedReader(new FileReader(f));
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        }
    }
    public FastScanner(InputStream f) {
        br = new BufferedReader(new InputStreamReader(f));
    }
    String next() {
        while (st == null || !st.hasMoreTokens()) {
            String s = null;
            try {
                s = br.readLine();
            } catch (IOException e) {
                e.printStackTrace();
            }
            if (s == null)
                return null;
            st = new StringTokenizer(s);
        }
        return st.nextToken();
    }
    boolean hasMoreTokens() {
        while (st == null || !st.hasMoreTokens()) {
            String s = null;
            try {
                s = br.readLine();
            } catch (IOException e) {
                e.printStackTrace();
            }
            if (s == null)
                return false;
            st = new StringTokenizer(s);
        }
        return true;
    }
    int nextInt() {
        return Integer.parseInt(next());
    }
    long nextLong() {
        return Long.parseLong(next());
    }
}