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 30
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 2
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 1
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 10
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 1
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import java.io.*;
import java.util.*;

public class Main {

	Scanner in;
	PrintWriter out;
	
	void solve() {
		int n = in.nextInt();
		boolean used1[] = new boolean[26];
		boolean used2[][] = new boolean[26][26];
		boolean used3[][][] = new boolean[26][26][26];
		
		Arrays.fill(used1, false);
		for (boolean ar[] : used2) {
			Arrays.fill(ar, false);
		}
		
		
		for (boolean arr[][] : used3) {
			for (boolean ar[] : arr) {
				Arrays.fill(ar, false);
			}
		}

		for (int i = 0; i < n; i++) {
			char s[] = in.next().toCharArray();
			used1[s[0] - 'a'] = true;
			if (s.length > 1) { 
				used1[s[1] - 'a'] = true;
				used2[s[0] - 'a'][s[1] - 'a'] = true;
				for (int j = 2; j < s.length; j++) {
					used1[s[j] - 'a'] = true;
					used2[s[j - 1] - 'a'][s[j] - 'a'] = true;
					used3[s[j - 2] - 'a'][s[j - 1] - 'a'][s[j] - 'a'] = true;
				}
			}
		}
		
		for (int i = 0; i < used1.length; i++) {
			if (!used1[i]) {
				out.println((char)(i + 'a'));
				return;
			}
		}
		
		for (int i = 0; i < used2.length; i++) {
			for (int j = 0; j < used2[i].length; j++) {
				if (!used2[i][j]) {
					out.print((char)(i + 'a'));
					out.print((char)(j + 'a'));
					return;
				}
			}
		}
		
		for (int i = 0; i < used3.length; i++) {
			for (int j = 0; j < used3[i].length; j++) {
				for (int k = 0; k < used3[i].length; k++) {
					if (!used3[i][j][k]) {
						out.print((char)(i + 'a'));
						out.print((char)(j + 'a'));
						out.print((char)(k + 'a'));
						return;
					}
				}
			}
		}
		throw new Error();
	}
	
	void run() {
		in = new Scanner(System.in);
		out = new PrintWriter(System.out);
		
		try {
			solve();
		} finally {
			out.close();
		}
	}
	
	public static void main(String[] args) {
		new Main().run();
	}
}
