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

public class Main {

    static BufferedReader br;
    static PrintWriter out;
    static StringTokenizer st;
    
    
    static String min(){
//      HashSet<String> used = new HashSet<String>();
        Queue<String> q = new LinkedList<String>();
        q.add("");
//      used.add("");
        while(!q.isEmpty()){
            String s = q.remove();
            if(!s.isEmpty()){
                boolean b = true;
                for(int i = 0; i < n; i++){
                    if(a[i].contains(s)){
                        b = false;
                    }
                }
                if(b){
                    return s;
                }
            }
            for(int i = 0; i < 26; i++){
                String newS = s + (char)('a' + i);
                q.add(newS);
            }
        }
        return "";
    }
    
    static int n;
    static String[] a;
    
    static void solve() throws Exception {
        n = nextInt();
        a = new String[n];
        for(int i = 0; i < n; i++){
            a[i] = next();
        }
        String ans = min();
        System.out.println(ans);
    }

    static int nextInt() throws IOException {
        return Integer.parseInt(next());
    }

    static long nextLong() throws IOException {
        return Long.parseLong(next());
    }

    static double nextDouble() throws IOException {
        return Double.parseDouble(next());
    }

    static String next() throws IOException {
        while (st == null || !st.hasMoreTokens()) {
            String line = br.readLine();
            if (line == null) {
                return null;
            }
            st = new StringTokenizer(line);
        }
        return st.nextToken();
    }

    public static void main(String[] args) {
        try {
            File file = new File(".in");
            InputStream input = System.in;
            OutputStream output = System.out;
            if (file.canRead()) {
                input = new FileInputStream(file);
                output = new FileOutputStream(".out");
            }
            br = new BufferedReader(new InputStreamReader(input));
            out = new PrintWriter(new PrintStream(output));
            solve();
            out.close();
            br.close();
        } catch (Throwable t) {
            t.printStackTrace();
        }
    }
}