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/**
 *
 * @author saurabh
 */
public class Cforces1 {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args)throws Exception {
        // TODO code application logic here
      java.io.BufferedReader br= new java.io.BufferedReader(new 
           java.io.InputStreamReader(System.in));
      int t=Integer.parseInt(br.readLine());
      int i,j,k,n;
      String str[]=new String[t];
      int ch[]=new int[26];
      for(i=0;i<26;i++)
          ch[i]=0;
      for(i=0;i<t;i++){
          str[i]=br.readLine();
          for(j=0;j<str[i].length();j++)
          {
              ch[(str[i].charAt(j))%97]++;
          }
      }
      StringBuffer sb=new StringBuffer("");
      int flag=0;
      char v='\0';
      for(i=0;i<26;i++)
      {
          if(ch[i]==0)
          {
              flag=1;
              System.out.println((char)(i+97));
              break;
          }
      }
      
      int y=0,l=0;
      int len=0;
      if(flag==0)
      {
          sb.append((char)(y+97)).append((char)(y+97));
          len=2;
          l=1;
          while(true)
          {
              int x=0;
              for(i=0;i<t;i++)
              {
                if(str[i].contains(sb)){
                break;
                }
                else
                {
                    x=1;
              //      System.out.println(i);
                    break;
                }
              }
              
              if(x==1)
                  break;
              
              y++;
              if(y==26)
              {
                  sb.deleteCharAt(l);
                  y=0;
                  sb.append((char)(y+97));
                  l--;
                  if(l==0){
                      sb.append((char)(97));
                      l=sb.toString().length()-1;
                  }
                  else
                      sb.replace(l, l,Character.toString((char)(((int)(sb.charAt(l))+1))));
                  
                  continue;
              }
              sb.deleteCharAt(l);
              sb.append((char)(y+97));
          }
          
          System.out.println(sb);
      }
    }
}
