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 2
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 1
b

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import java.io.*;
import java.util.*;

public class pr170B implements Runnable {

	BufferedReader in;
	PrintWriter out;
	StringTokenizer str;

	public void solve() throws IOException {
		int n = nextInt();
		String[] s = new String[n];
		for (int i = 0; i < n; i++) {
			s[i] = nextToken();
		}
		String ans = "";
		boolean f = false;
		for (char i = 'a'; i <= 'z' && !f; i++) {
			f = true;
			for (int x = 0; x < n; x++) {
				if (s[x].contains(i + "")) {
					f = false;
				}
			}
			if (f)
				ans = i + "";
		}
		if (ans.length() < 1) {
			f = false;
			for (char i = 'a'; i <= 'z' && !f; i++) {
				for (char j = 'a'; j <= 'z' && !f; j++) {
					f = true;
					for (int x = 0; x < n; x++) {
						if (s[x].contains(i + "" + j)) {
							f = false;
						}
					}
					if (f)
						ans = i + "" + j;
				}
			}
		}
		if (ans.length() < 1) {
			f = false;
			for (char i = 'a'; i <= 'z' && !f; i++) {
				for (char j = 'a'; j <= 'z' && !f; j++) {
					for (char k = 'a'; k <= 'z' && !f; k++) {
						f = true;
						for (int x = 0; x < n; x++) {
							if (s[x].contains(i + "" + j + "" + k)) {
								f = false;
							}
						}
						if (f)
							ans = i + "" + j + "" + k;
					}
				}
			}
		}
		out.println(ans);
	}

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

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

	public double nextDouble() throws IOException {
		return Double.parseDouble(nextToken());
	}

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

	public void run() {
		try {
			in = new BufferedReader(new InputStreamReader(System.in));
			out = new PrintWriter(System.out);
			solve();
			in.close();
			out.close();
		} catch (IOException e) {
		}
	}

	public static void main(String[] args) {
		new Thread(new pr170B()).start();
	}
}
