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

import static java.lang.Math.*;
import static java.util.Arrays.*;
import static java.util.Collections.*;

public class  test{
	
	// ArrayList<Integer> lis = new ArrayList<Integer>();
	// ArrayList<String> lis = new ArrayList<String>();
	//  PriorityQueue<P> que = new PriorityQueue<P>();
	// PriorityQueue<Integer> que = new PriorityQueue<Integer>();
	//  Stack<Integer> que = new Stack<Integer>();
    //	static long sum=0;
	// 1000000007 (10^9+7)
	static int mod = 1000000007;
	//static int mod = 1000000009,r=0;
   // static int dx[]={1,-1,0,0};
//	static int dy[]={0,0,1,-1};
//	static int dx[]={1,-1,0,0,1,1,-1,-1};
//  static int dy[]={0,0,1,-1,1,-1,1,-1};
	//static long H,L;
	//static Set<Integer> set = new HashSet<Integer>();

	static String al,f[];
    	
	
public  static void main(String[] args)   throws Exception, IOException{
   //String line=""; throws Exception, IOException
   //(line=br.readLine())!=null
	//Scanner sc =new Scanner(System.in);
	// !!caution!! int long //  
	Reader sc = new Reader(System.in);
  // while( ){
	  // int  n=sc.nextInt(),m=sc.nextInt();//a[]=new int[n],b[]=new int[n];
	int   n=sc.nextInt(),r=0;
	al="abcdefghijklmnopqrstuvwxyz";
    f=new String[n];
    
    for(int i=0;i<n;i++){f[i]=sc.nextString(); }

    
    System.out.println(rep(""));
    
}

 static String  rep(String x){
	int k=0; 
	 for(int i=0;i<26;i++){
		 String s=x+al.charAt(i);
		for(int t=0;t<f.length;t++){
			if( f[t].contains(s) ){k++;break;}
		}
	   if(0==k)return s;
	   k--;
	 //  db(s);
	 }
	 return rep(x+'a'); 
 }


static void db(Object... os){
    System.err.println(Arrays.deepToString(os));

}


}

class Reader
{
	private BufferedReader x;
	private StringTokenizer st;
	
	public Reader(InputStream in)
	{
		x = new BufferedReader(new InputStreamReader(in));
		st = null;
	}
	public String nextString() throws IOException
	{
		while( st==null || !st.hasMoreTokens() )
			st = new StringTokenizer(x.readLine());
		return st.nextToken();
	}
	public int nextInt() throws IOException
	{
		return Integer.parseInt(nextString());
	}
	public long nextLong() throws IOException
	{
		return Long.parseLong(nextString());
	}
	public double nextDouble() throws IOException
	{
		return Double.parseDouble(nextString());
	}
}

/*

class P implements Comparable<P>{
	int x,y;
	P(int x,int y){ this.x=x; this.y=y;  }
	
	public int compareTo(P z) {
	 if( x-z.x!=0)   return  x-z.x ;  //ascend 
	 else return  y-z.y ; 
	}
}
//*/
