3
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 1
z
 30
b
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n
f
m
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j
k
p
h
a
c
w
z
z
p
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d
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 5
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 2
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 5
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abcdefghijklmnopqrst
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 2
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 5
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 1
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 10
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 5
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 2
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 1
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 30
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 30
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 1
b

a
 a
 e
 a
 a
 y
 b
 aa
 a
 ab
 z
 e
 b
 aa
 l
 a


/* BY - LAXMIKANT KATRE (laxmikant.katre) */

import java.io.*;
import java.util.*;
 
public class Main {
    
    public static void main(String[] args) {
        
        MyScanner sc = new MyScanner();
        out = new PrintWriter(new BufferedOutputStream(System.out));
 
        int n = sc.nextInt();
        
        String str = "";
        String ans = "";
        boolean f = false;
        
        for(int i = 0; i < n; ++i) {
            String in = sc.next();
            str += in + ")";
        }
        
        for(int i = 'a'; i <= 'z'; ++i) {
            if(!str.contains("" + (char)i)) {
                f = true;
                out.println((char)i);
                break;
            }
        }
        
        
        if(!f) {
            String in = "";
            for(int i = 'a'; i <= 'z'; ++i) {
                for(int j = 'a'; j <= 'z'; ++j) {
                    in = (char)i + "";
                    in += (char)j;
                    if(!str.contains(in)) {
                        out.println(in);
                        f = true;
                        break;
                    }
                }
                if(f) break;
            }
        }
       
        out.close();
 
        /*
        int n      = sc.nextInt();        // read input as integer
        long k     = sc.nextLong();       // read input as long
        double d   = sc.nextDouble();     // read input as double
        String str = sc.next();           // read input as String
        String s   = sc.nextLine();       // read whole line as String
 
        int result = 40*n;
        out.println(result);                    // print via PrintWriter
        */

    }

    //-----------PrintWriter for faster output---------------------------------
    public static PrintWriter out;

    //-----------MyScanner class for faster input----------
    public static class MyScanner {
        BufferedReader br;
        StringTokenizer st;
 
        public MyScanner() {
            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());
        }

        String nextLine(){
            String str = "";
            try {
                str = br.readLine();
            } catch (IOException e) {
                e.printStackTrace();
            }
            return str;
        }
    }
    //--------------------------------------------------------
}