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 2
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 2
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 5
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 1
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 10
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 1
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 1
b

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import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Set;
import java.util.StringTokenizer;

public class B {
    private static BufferedReader in;
    private static StringTokenizer st;
    private static PrintWriter out;
    private static int cnt;
    
    public static void main(String[] args) throws IOException {
        in = new BufferedReader(new InputStreamReader(System.in));
        st = new StringTokenizer("");
        out = new PrintWriter(new BufferedWriter(new OutputStreamWriter(System.out)));
        int n = nextInt();
        String lex[] = new String [7000];
        cnt=0;
        for (int i = 'a'; i <='z'; i++) {
            lex[cnt] = "";
            lex[cnt++] += (char) i;
        }
        for (int i = 'a'; i <='z'; i++) {
            for (int j = 'a'; j <='z'; j++) {
                lex[cnt] = "";
                lex[cnt++] += (char)(i) + ""+(char)(j);
            }
        }
        String s ="";
        for (int i = 0; i < n; i++) {
            s += next()+"#";
        }
        for (int i = 0; i < cnt; i++) {
            if(!s.contains(lex[i])){
                System.out.println(lex[i]);
                return;
            }
        }
        
    }
    static String next() throws IOException{
        while(!st.hasMoreTokens()){
            st = new StringTokenizer(in.readLine());
        }
        return st.nextToken();
    }
    static int nextInt() throws NumberFormatException, IOException{
        return Integer.parseInt(next());
    }
    static long nextLong() throws NumberFormatException, IOException{
        return Long.parseLong(next());
    }
    
    static double nextDouble() throws NumberFormatException, IOException{
        return Double.parseDouble(next());
    }

}
