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.util.*;
import java.io.*;

public class File {
    public static class FastScanner {
        BufferedReader br;
        StringTokenizer st;

        public FastScanner() {
            br = new BufferedReader(new InputStreamReader(System.in));
        }

        String next() {
            while (st == null || !st.hasMoreElements()) {
                try {
                    st = new StringTokenizer(br.readLine());
                }
                catch (IOException e) {
                    e.printStackTrace();
                }
            }

            return st.nextToken();
        }

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

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

        double nextDouble() {
            return Double.parseDouble(next());
        }
    }
	
	public static class Trie {
		TrieNode root;
		
		public Trie () {
			root = new TrieNode();
		}
		
		public void add(String word) {
			TrieNode curr = root;
			
			for (char c : word.toCharArray()) {
				if (curr.children[c-'a'] == null) {
					curr.children[c-'a'] = new TrieNode();
				}
				
				curr = curr.children[c-'a'];
			}
		}
		
		public boolean contains(String word) {
			TrieNode curr = root;
			
			for (char c : word.toCharArray()) {
				if (curr.children[c-'a'] == null) {
					return false;
				}
				
				curr = curr.children[c-'a'];
			}
			
			return true;
		}
	}
	
	public static class TrieNode {
		TrieNode[] children = new TrieNode[26];
	}
	
    public static void main(String[] args) {
        FastScanner sc = new FastScanner();
        PrintWriter out = new PrintWriter(new BufferedOutputStream(System.out));
		
		
		
		
		Trie trie = new Trie();
		
		int n = sc.nextInt();
		
		for (int i = 0; i < n; i++) {
			String word = sc.next();
			
			for (int j = 0; j < word.length(); j++) {
				trie.add(word.substring(j, j+1));
				
				if (j + 1 < word.length()) {
					trie.add(word.substring(j, j+2));
				}
			}
		}
		
		boolean found = false;
		String res = "";
		
		// Try all length 1
		for (char c = 'a'; c <= 'z'; c++) {
			if (!trie.contains("" + c)) {
				res = "" + c;
				found = true;
				break;
			}
		}
		
		
		// Try all length 2
		if (!found) {
			for (char c = 'a'; c <= 'z'; c++) {
				String sub = "a" + c;
				
				if (!trie.contains(sub)) {
					res = sub;
					found = true;
					break;
				}
			}
		}
		
		
		out.println(res);
		
		
		
		
		
        out.close();
    }
}













