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 1
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 30
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 5
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 2
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 5
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 2
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 5
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 1
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 10
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 5
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 2
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 1
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 30
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 30
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 1
b

a
 a
 e
 a
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 y
 b
 aa
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 ab
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 a

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.HashSet;

public class Main {

	public static void main(String[] args) throws NumberFormatException,
			IOException {
		BufferedReader bufer = new BufferedReader(new InputStreamReader(
				System.in));
		int n = Integer.parseInt(bufer.readLine());
		HashSet<String> tabla = new HashSet<String>();
		String arr[] = new String[n];
		for (int i = 0; i < n; i++) {
			arr[i] = bufer.readLine();
		}
		int tam = 0;
		char c[] = new char[21];
		c[0] = 'a';
		String inicial = "a";
		String res="";
		while (true) {
			boolean x = false;
			inicial = "";
			for (int i = 0; i < tam+1; i++) {
				inicial += c[i] + "";
			}
			for (int i = 0; i < arr.length; i++) {
				if (arr[i].indexOf(inicial) != -1) {
					x = true;
					break;
				}

			}
			if (x) {
				if (c[tam] == 'z') {
					int sum = 0;
					for (int i = tam; i >= 0; i--) {
						if (c[i] == 'z')
							sum = i;
						else {
							break;
						}
					}
					if (sum != 0) {
						for (int i = sum; i < tam+1; i++) {
							c[i] = 'a';
						}
						c[sum - 1]++;
					} else {
						for (int i = 0; i < tam+1; i++) {
							c[i]='a';
						}
						tam++;
						c[tam]='a';
					}

				}else{
					c[tam]++;
				}
			}else{
				res=inicial;
				break;
			}
		}
		System.out.println(res);

	}

}
