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

public class Main {

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        PrintWriter out = new PrintWriter(System.out);
        int n = in.nextInt();
        String[] a = new String[n];
        for (int i = 0; i < n; i++) {
            a[i] = in.next();
        }
        String ans = "";
        HashSet<String> set = new HashSet<String>();
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < a[i].length(); j++) {
                for (int k = j; k < a[i].length(); k++) {
                    set.add(a[i].substring(j, k + 1));
                }
            }
        }
        boolean bl = true;
        
        for(int i = 0; i < 26 && bl; i++){
                        if (!set.contains(String.valueOf((char)(i + (int)'a')))){
                ans = String.valueOf((char)(i + (int)'a'));
                bl = false;
            }
        }
        for (int i = 0; i < 26 && bl;  i++) {

            ans = String.valueOf((char)(i + (int)'a'));
            for (int j = 0; j < 26 && bl; j++) {
                if (!set.contains(ans + String.valueOf((char)(j + (int)'a')) )){
                    ans +=  String.valueOf((char)(j + (int)'a')) ;
                    bl = false;
                }
            }
        }
        out.println(ans);
        out.close();
    }
}
