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 30
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 2
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 5
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 2
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 1
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 10
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 2
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 1
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import java.util.HashMap;
import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = Integer.parseInt(sc.nextLine());
        String map="";
        HashMap<Character, Integer> L = new HashMap<Character, Integer>();
        for (int i = 0; i < n; i++) {
            String str = sc.nextLine();
            map+=str+" ";
            for (int j = 0; j < str.length(); j++) {
                L.put(str.charAt(j), 0);
            }
        }
        int size = L.size();
        if (size == 26) {
            for (int l = 97; l < 123; l++) {
                char ll=(char)l;
                boolean b=false;
                for (int i = 97; i < 123; i++) {
                    char cc = (char) i;
                    if (!map.contains(ll+""+cc)) {
                        System.out.println(ll+""+cc);
                        b=true;
                        break;
                    }
                }
                if(b)break;
            }
        } else {
            for (int i = 97; i < 123; i++) {
                char cc = (char) i;
                if (!L.containsKey(cc)) {
                    System.out.println(cc);
                    break;
                }
            }
        }
    }
}