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 1
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 2
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 2
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 5
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 1
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 10
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 2
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 1
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 1
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import java.io.*;
import java.util.*;

/**
  * Built using my Brain
  * Actual solution is at the bottom
  *
  * @author Lenard Hoffstader
*/

public class cfjava
{
  public static void main(String[] args)
  {
    OutputStream outputStream = System.out;
    FastReader in = new FastReader();
    PrintWriter out = new PrintWriter(outputStream);
    PROBLEM solver = new PROBLEM();
    int t=1;
    // t = in.nextInt();
    for(int i=0;i<t;i++){
      solver.solve(in, out);
    }
    out.close();
  }
  static class PROBLEM
  {
    static TreeSet<String> ts;
    public void solve(FastReader in,PrintWriter out)
    {
      int n = in.nextInt();
      ts =  new TreeSet<>();
      for(int i=0;i<n;i++){
        genSubstrings(in.nextLine());
      }
      Queue<String> q = new LinkedList<>();
      for(char ch='a';ch<='z';ch++){
        String s = "";s+=ch;
        q.add(s);
      }
      while(true){
        String s = q.poll();
        if(ts.contains(s)){
          for(char ch='a';ch<='z';ch++){
            String s1 = s;s1+=ch;
            q.add(s1);
          }
        }
        else {
          out.println(s);
          return;
        }
      }
    }
    static void genSubstrings(String s) {
      int len = s.length();
      for(int i=0;i<len;i++){
        String sub = "";
        for(int j=i;j<len;j++){
          sub+=s.charAt(j);
          ts.add(sub);
        }
      }
    }
  }
  
  static class FastReader 
  {
    BufferedReader br; 
    StringTokenizer st;

    public FastReader() 
    { 
      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());}

    char nextChar(){return next().charAt(0);}

    boolean nextBoolean(){return !(nextInt()==0);}
    // boolean nextBoolean(){return Boolean.parseBoolean(next());}

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