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 2
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import java.io.*;

public class CF278B {
	public static void main(String[] args) throws IOException {
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		int n = Integer.parseInt(br.readLine());
		boolean[] a1 = new boolean[26];
		boolean[][] a2 = new boolean[26][26];
		while (n-- > 0) {
			char[] cc = br.readLine().toCharArray();
			for (int i = 0; i < cc.length; i++) {
				int c = cc[i] - 'a';
				a1[c] = true;
				if (i > 0) {
					int b = cc[i - 1] - 'a';
					a2[b][c] = true;
				}
			}
		}
		for (int c = 0; c < 26; c++)
			if (!a1[c]) {
				System.out.println((char) (c + 'a'));
				return;
			}
		for (int b = 0; b < 26; b++)
			for (int c = 0; c < 26; c++)
				if (!a2[b][c]) {
					System.out.print((char) (b + 'a'));
					System.out.println((char) (c + 'a'));
					return;
				}
	}
}
