3
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 1
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 30
b
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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

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

public class Main {

    
    static class name implements Comparable<name>{
        String name;

        public name(String name) {
            super();
            this.name = name;
        }

        @Override
        public int compareTo(name o) {
            if(name.length()-o.name.length()==0) return name.compareTo(o.name);
            return name.length()-o.name.length();
        }
        
    }
    
    static String sum1(String w){
        String ans="";
        int i;
        char c='*';
        for(i=w.length()-1;i>=0;i--){
            c = (char)(((w.charAt(i)-'a'+1)%26)+97);            
            ans=c+ans;
            if(c!='z') {i--;break;}
            
        }
        if(c=='a') ans="a"+ans;
        for(;i>=0;i--) ans=w.charAt(i)+ans;
        return ans;
    }
    
    public static void main(String[] args) throws Exception {
        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
        ArrayList<name> sub = new ArrayList<name>();
        HashSet<String> c = new HashSet<String>();
        for(int i=0;i<n;i++){
            String line = in.next();
            for(int j=0;j<line.length();j++)
                for(int k=j;k<line.length();k++){
                    String name = line.substring(j, k+1);
                    if(!c.contains(name)){
                        c.add(name);
                        sub.add(new name(name));
                    }
                }
        }
        String init="a";
        Collections.sort(sub);
        for(int i=0;i<sub.size();i++){
            if(init.compareTo(sub.get(i).name)<0){
                System.out.println(init);
                return;
            }
            init = sum1(init);
        }
        System.out.println(init);
            
        
    }
}