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import java.util.Scanner;


public class my_class {
	private static int num_string;
	private static String array[];
	public static boolean my_func(String my_string){
		char my_char = 'a';
		int i;
		for(char sim = 'a';sim < (my_char + 26);sim++){
			for(i = 0;i < num_string;i++){
				if(array[i].contains(String.valueOf(my_string + sim))){
					break;
				}
			}
			if(i == num_string){
				System.out.println(my_string + sim);
				return true;
			}
		}
		for(char a = 'a';a < ('a' + 26);a++){
			if(my_func(my_string + a)){
				return true;
			}
		}
		return false;
	}
	
	public static void main(String[] args) {
		Scanner ololo = new Scanner(System.in);
		num_string = ololo.nextInt();
		boolean my_bool = false;
		int i;
		char my_sim = 'a';
		ololo.nextLine();
		array = new String[num_string];
		for(i = 0;i < num_string;i++){
			array[i] = ololo.nextLine();
		}
		char my_char = 'a';
		for(char sim = 'a';sim < (my_char + 26);sim++){
			for(i = 0;i < num_string;i++){
				if(array[i].contains(String.valueOf(sim))){
					break;
				}
			}
			if(i == num_string){
				my_bool = true;
				my_sim = sim;
				break;
			}
		}
		if(my_bool){
			System.out.println(my_sim);
		}
		else{
			for(char a = 'a';a < ('a' + 26);a++){
				if(my_func(String.valueOf(a))){
					break;
				}
			}
		}
	}	
}
