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
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pmfm
 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
gtpr
kuzoxmiixxbl
crgqtuo
jhbplhpklrgwnaugdf
zuxdaat
aycv
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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gecntpcmoojfwp
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jrsrvsvbaiumlmkptn
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ktvyzvddgllht
szahigtmklglrcocbo
znligfxkgxzkcfeu
liryvzmqwhr
xgrxkgiehxztv
etrjxdczppafly
jrdgajschhwsci
doxnxbjwptnimjmsuijx
zciwkbvrhgsjhrr
ehh
bzlnmd
xpbtbpftimnn
khbknqbvdkdd
mldj
skrvnyz
avptgpjmenfzfxwckmt
 30
e
x
itaubd
xcn
v
x
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ci
aqs
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jos
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ge
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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.*;
import java.math.*;
import java.text.*;

public class Main {

	int n;
	StringBuilder str = new StringBuilder();
	Set<String> set = new TreeSet<String>();
	
	void Next() {
		int len = str.length();
		char c = str.charAt(len - 1);
		if (c == 'z') {
			str.setCharAt(len - 1, 'a');
			str.append('a');
		} else {
			str.setCharAt(len - 1, ++c);
		}
	}
	
	void run() throws Exception {
		n = cin.nextInt();
		for (int i = 0; i < n; i++) {
			str.append(cin.next());
			int len = str.length();
			for (int j = 0; j < len; j++) {
				for (int k = j + 1; k <= len; k++) {
					set.add(str.substring(j, k));
				}
			}
			str.delete(0, len);
		}
		str.append("a");
		boolean flag = false;
		StringBuilder ss = new StringBuilder();
		while (true) {
			ss = str;
			if (set.contains(ss.toString())) {
				Next();
			} else {
				flag = true;
				System.out.println(str);
				return;
			}
		}	
	}  
	
	public static void main(String[] args) throws Exception {	
//		InputStreamReader is = new InputStreamReader(new FileInputStream("E:/input.txt"));
//		BufferedReader cin = new BufferedReader(is);
		
		new Main().run();
//		out.close();
	}
	
//	InputStreamReader is = new InputStreamReader(System.in);
//	BufferedReader cin = new BufferedReader(is);
	
	static InputStream inputStream = System.in;
	static InputReader cin = new InputReader(inputStream);
//	static OutputStream outputStream = System.out;
//	static OutputWriter out = new OutputWriter(outputStream);
	
//	Scanner cin = new Scanner(new BufferedInputStream(System.in));
	
}

class InputReader { 
    private InputStream stream; 
    private byte[] buf = new byte[1024]; 
    private int curChar; 
    private int numChars; 

    public InputReader(InputStream stream) { 
        this.stream = stream; 
    } 

	public int read() { 
        if (numChars == -1) 
            return -1; 
            //throw new InputMismatchException(); 
        if (curChar >= numChars) { 
            curChar = 0; 
            try { 
                numChars = stream.read(buf); 
            } catch (IOException e) { 
                throw new InputMismatchException(); 
            } 
            if (numChars <= 0) 
                return -1; 
        } 
        return buf[curChar++]; 
    } 

    public int nextInt() { 
        int c = read(); 
        if (c == -1) 
            return -1; 
        while (isSpaceChar(c)) 
            c = read(); 
        int sgn = 1; 
        if (c == '-') { 
            sgn = -1; 
            c = read(); 
        } 
        int res = 0; 
        do { 
            if (c < '0' || c > '9') 
                throw new InputMismatchException(); 
            res *= 10; 
            res += c - '0'; 
            c = read(); 
        } while (!isSpaceChar(c)); 
        return res * sgn; 
    } 
    
    public long nextLong() {
    	int c = read(); 
        if (c == -1) 
            return -1; 
        while (isSpaceChar(c)) 
            c = read(); 
        int sgn = 1; 
        if (c == '-') { 
            sgn = -1; 
            c = read(); 
        } 
        long res = 0; 
        do { 
            if (c < '0' || c > '9') 
                throw new InputMismatchException(); 
            res *= 10; 
            res += c - '0'; 
            c = read(); 
        } while (!isSpaceChar(c)); 
        return res * sgn;
    }
     
    public String next() {   
        StringBuilder str = new StringBuilder();   
        int ch;   
        while (isSpaceChar(ch = read()));   
        if (ch == -1)   
            return null;   
        do {   
            str.appendCodePoint(ch);   
        } while (!isSpaceChar(ch = read()));   
        return str.toString();   
    }  
     
    public static boolean isSpaceChar(int c) { 
        return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1; 
    } 

    public char nextCharacter() { 
        int c = read(); 
        while (isSpaceChar(c)) 
            c = read(); 
        return (char) c; 
    }   
} 


class OutputWriter {
	private final PrintWriter writer;

	public OutputWriter(OutputStream outputStream) {
		writer = new PrintWriter(new BufferedWriter(new OutputStreamWriter(outputStream)));
	}

	public OutputWriter(Writer writer) {
		this.writer = new PrintWriter(writer);
	}

	public void print(Object...objects) {
		for (int i = 0; i < objects.length; i++) {
			if (i != 0)
				writer.print(' ');
			writer.print(objects[i]);
		}
	}

    public void printLine(Object...objects) {
		print(objects);
		writer.println();
	}

	public void close() {
		writer.close();
	}

	}