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import java.io.*;
import java.math.BigInteger;
import java.util.*;
import java.util.concurrent.ArrayBlockingQueue;


public class B
{
        String line;
        StringTokenizer inputParser;
        BufferedReader is;
        FileInputStream fstream;
        DataInputStream in;
        String FInput="";
        
        void openInput(String file)
        {

                if(file==null)is = new BufferedReader(new InputStreamReader(System.in));//stdin
                else
                {
                        try{
                
                                
                        fstream = new FileInputStream(file);
                        in = new DataInputStream(fstream);
                        is = new BufferedReader(new InputStreamReader(in));
                        }catch(Exception e)
                        {
                                System.err.println(e);
                        }
                }

        }
        
        void readNextLine()
    	{
    		try {
    			line = is.readLine();
    			inputParser = new StringTokenizer(line, " ");
    			//System.err.println("Input: " + line);
    		} catch (IOException e) {
    			System.err.println("Unexpected IO ERROR: " + e);
    		}	
    		catch (NullPointerException e)
    		{
    			line=null;
    			
    		}
    		
    	}
        
        long NextLong()
        {
                String n = inputParser.nextToken();
                
                long val = Long.parseLong(n);
                
                 return val;
        }
        
        int NextInt()
        {
                String n = inputParser.nextToken();
                int val = Integer.parseInt(n);
                
                //System.out.println("I read this number: " + val);
                return val;
        }
        
        String NextString()
        {
                String n = inputParser.nextToken();
                return n;
        }
        
        void closeInput()
        {
                try {
                        is.close();
                } catch (IOException e) {
                        System.err.println("Unexpected IO ERROR: " + e);
                }
                        
        }
        
        
        public static void main(String [] argv)
        {
                //String filePath="input.txt";
        		//String filePath="D:\\_d\\learn\\coursera\\algorithms and design I\\data\\SCC.txt";
                String filePath=null;
                if(argv.length>0)filePath=argv[0];
                new B(filePath);
        }
        
        public void readFInput()
    	{
    		for(;;)
    		{
    			try
    			{
    				readNextLine();
    				FInput+=line+" ";
    			}
    			catch(Exception e)
    			{
    				break;
    			}
    		}
    		inputParser = new StringTokenizer(FInput, " ");
    	}
        
        
        public B(String inputFile)
        {
        	openInput(inputFile);
        	
        	
        	readNextLine();
        	int N=NextInt();
        	String [] p = new String [N];
        	
        	
        	for(int i=0; i<N; i++)
        	{
        		readNextLine();
        		p[i]=line;
        		
        	}
        	String ret="";
        	
        	for(int i=0; i<26&&ret.length()==0; i++)
        	{
        		String s=""+(char)('a'+i);
        		boolean ok=true;
        		for(int j=0; j<N&&ok; j++)
        			if(p[j].indexOf(s)>-1)ok=false;
        		if(ok)ret=s;
        	}
        	
        	for(int i=0; i<26&&ret.length()==0; i++)
        		for(int ii=0; ii<26&&ret.length()==0; ii++)	        	
	        	{
	        		String s=""+(char)('a'+i)+(char)('a'+ii);
	        		boolean ok=true;
	        		for(int j=0; j<N&&ok; j++)
	        			if(p[j].indexOf(s)>-1)ok=false;
	        		if(ok)ret=s;
	        	}
        	
        	for(int i=0; i<26&&ret.length()==0; i++)
	        	for(int ii=0; ii<26&&ret.length()==0; ii++)
	        		for(int iii=0; iii<26&&ret.length()==0; iii++)
	            	{
	            		String s=""+(char)('a'+i)+(char)('a'+ii)+(char)('a'+iii);
	            		boolean ok=true;
	            		for(int j=0; j<N&&ok; j++)
	            			if(p[j].indexOf(s)>-1)ok=false;
	            		if(ok)ret=s;
	            	}
        	for(int i=0; i<26&&ret.length()==0; i++)
        		for(int ii=0; ii<26&&ret.length()==0; ii++)
        			for(int iii=0; iii<26&&ret.length()==0; iii++)
		        		for(int iiii=0; iiii<26&&ret.length()==0; iiii++)
		            	{
		            		String s=""+(char)('a'+i)+(char)('a'+ii)+(char)('a'+iii)+(char)('a'+iiii);
		            		boolean ok=true;
		            		for(int j=0; j<N&&ok; j++)
		            			if(p[j].indexOf(s)>-1)ok=false;
		            		if(ok)ret=s;
		            	}
	        	
        	System.out.println(ret);
        	
            closeInput();		
    	}
      


		public static void out(Object s)
        {
        try{
	        FileWriter fstream = new FileWriter("output.txt");
	        BufferedWriter out = new BufferedWriter(fstream);
	        out.write(s.toString());
	        out.close();
	        }catch (Exception e){
	        	System.err.println("Error: " + e.getMessage());
	        }
        }
               
}