3
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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
 a
 ab
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 a

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


public class B170 {

	public static void main (String [] args) throws Exception {
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		
		int n = Integer.parseInt(br.readLine());
		
		
		HashSet<String> substrings = new HashSet<String>();
		
		String s;
		for (int i = 0 ; i < n; i++) {
			s = br.readLine();
			for (int l = 0 ; l <= s.length(); l++)
				for ( int pos = 0; pos < s.length() - l; pos++)
					substrings.add(s.substring(pos, pos + l + 1));
		}
		
		boolean found = false;
		
		LinkedList<String> queue = new LinkedList<String>();
		for (char c = 'a'; c <= 'z'; c++)
			queue.add("" + c);
		
		String sol;
		while (true) {
			String top = queue.poll();
			if (!substrings.contains(top)) {
				sol = top;
				break;
			} else {
				for (char c = 'a'; c <= 'z'; c++)
					queue.add(top + c);
			}
		}
		
		System.out.println(sol);
		
		
		br.close();
		
	}
}
