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 2
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 5
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import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.StringTokenizer;

/**
 * Created by ahmadi on 6/29/18.
 */
public class Main {
    public static void main(String[] args) {
        BufferedReader br = new BufferedReader(
                new InputStreamReader(System.in));
        StringTokenizer st;
        try {
            st = new StringTokenizer(br.readLine());
            int n = Integer.parseInt(st.nextToken());
            ArrayList<String> arrayList = new ArrayList<String>();
            for (int i = 0; i < n; i++){
                st = new StringTokenizer(br.readLine());
                arrayList.add(st.nextToken());
            }
            int len = 1;
            int changing = 1;
            char[] name = new char[1];
            name[0] = 'a';
            while(true){
                if (changing == 0){
                    name = new char[len+1];
                    for (int i = 0; i <= len; i++)
                        name[i] = 'a';
                    len++;
                    changing = len;
                }
                if (name[changing-1] == 'z' && changing ==len){
                    changing--;
                    continue;
                }
                boolean flag = false;
                for (String anArrayList : arrayList)
                    if (anArrayList.contains(String.valueOf(name))) {
                        name[changing - 1]++;
                        flag = true;
                        break;
                    }
                if (!flag)
                    break;
                if(changing < len){
                    if(name[changing-1] == 'z'){
                        changing--;
                    }
                    else{
                        for (int i = changing; i < name.length; i++)
                            name[i]='a';
                        name[changing-1]++;
                        changing++;
                    }
                }

            }
            System.out.println(String.valueOf(name));
        } catch (IOException e) {
        }

    }
}