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

public class taskB {

    Scanner in;
    PrintWriter out;

    void solve() {
        int n = in.nextInt();
        String s[] = new String[n];
        in.nextLine();
        for (int i = 0; i < n; i++) {
            s[i] = in.nextLine();
        }
        StringBuilder lex[] = new StringBuilder[20000];
        lex[1] = new StringBuilder("a");
        for (int i = 1; i <= 26; i++) {
            char next = (char) ('a' + i - 1);
            lex[i] = new StringBuilder("");
            lex[i].append(next);
        }
        int pos = 27;
        for (int i = 1; i <= 26; i++) {
            for (int j = 1; j <= 26; j++) {
                lex[pos] = new StringBuilder("");
                lex[pos].append(lex[i]);
                lex[pos].append(lex[j]);
                pos++;
            }
        }
        for (int i = 1; i <= 26; i++) {
            for (int j = 1; j <= 26; j++) {
                for (int k = 1; k <= 26; k++) {
                    lex[pos] = new StringBuilder("");
                    lex[pos].append(lex[i]);
                    lex[pos].append(lex[j]);
                    lex[pos].append(lex[k]);
                    pos++;
                }
            }
        }
        for (int i = 1; i <= pos - 1; i++) {
            String search = lex[i].toString();
            boolean find = false;
            for (int j = 0; j < n; j++) {
                if (s[j].indexOf(search) != -1) {
                    find = true;
                    break;
                }
            }
            if (!find) {
                out.println(lex[i]);
                return;
            }
        }
    }

    void run() {
        in = new Scanner(System.in);
        out = new PrintWriter(System.out);
        try {
            solve();
        } finally {
            out.close();
        }
    }

    public static void main(String[] args) {
        new taskB().run();
    }
}
