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import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

/**
 *
 * @author dell
 */
public class slfkmse {
     public static void main(String args[]) throws IOException{
            BufferedReader b = new BufferedReader(new InputStreamReader(System.in));
            int  n =Integer.parseInt(b.readLine());
            String s[] = new String [n];
            for(int i=0;i<n;i++){
                s[i] = b.readLine();
            }
            for(int i=0;;i++){
                String str="";
                int j;
                int temp=i;
                char c;
                int count=0;
                do{
                    if(count==0){
                        c=(char)((temp%26)+97);
                    }else{
                        c=(char)((temp%26)+96);
                    }
                    count++;
                    str=c+str;
                    temp=temp/26;
                }while(temp!=0);
                for(j=0;j<n;j++){
                    if(s[j].contains(str))
                    break;
                }
                if(j==n){
                    System.out.println(str);
                    break;
                }
            }
     }       
}
