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 2
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 2
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 1
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 1
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import java.io.BufferedReader;
import java.io.File;
import java.io.FileReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.StringTokenizer;

public class B {
	BufferedReader in;
	StringTokenizer st;
	PrintWriter out;

	public void Solution() throws IOException {
		int n = nextInt();
		String a[] = new String[n];
		for (int i = 0; i < n; i++) {
			a[i] = nextToken();
		}
		String ans = "";
		for (int i = 0; i <= 25; i++) {
			ans = String.valueOf(((char) ((int) ('a') + i)));
			int res = 0;
			for (int j = 0; j < n; j++) {
				if (!a[j].contains(ans)) {
					res++;
				}
			}
			if (res == n) {
				System.out.println(ans);
				System.exit(0);
			}
		}
		ans = "";
		for (int i = 0; i <= 676; i++) {
			ans = String.valueOf(((char) ((int) ('a') + (i / 26))));
			ans += String.valueOf(((char) ((int) ('a') + ((i % 26)))));
			int res = 0;
			for (int j = 0; j < n; j++) {
				if (!a[j].contains(ans)) {
					res++;
				}
			}
			if (res == n) {
				System.out.println(ans);
				System.exit(0);
			}
		}
	}

	public void run() throws IOException {
		try {
			// in = new BufferedReader(new FileReader(new File("input.txt")));
			// out = new PrintWriter(new File("output.txt"));
			in = new BufferedReader(new InputStreamReader(System.in));
			out = new PrintWriter(System.out);
			Solution();
			out.close();

		} catch (Exception e) {
			e.printStackTrace();
			System.exit(261);
		}
	}

	public String nextToken() throws IOException {
		if (st == null || !st.hasMoreTokens()) {
			st = new StringTokenizer(in.readLine());
		}
		return st.nextToken();
	}

	public long nextlong() throws IOException {
		return Long.parseLong(nextToken());
	}

	public int nextInt() throws IOException {
		return Integer.parseInt(nextToken());
	}

	public static void main(String args[]) throws IOException {
		new B().run();
	}
}