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
xlaxwpjabtpwddc
txwdjmohrrszorrnomc
 1
a
 30
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 30
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 1
b

a
 a
 e
 a
 a
 y
 b
 aa
 a
 ab
 z
 e
 b
 aa
 l
 a

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.Arrays;
import java.util.HashSet;
import java.util.Set;

public class B {
	public static BufferedReader in;
	public static PrintWriter out;

	public static void main(String[] args) throws IOException {
		in = new BufferedReader(new InputStreamReader(System.in));
		out = new PrintWriter(System.out);

		boolean showLineError = true;
		if (showLineError) {
			solve();
			out.close();
		} else {
			try {
				solve();
			} catch (Exception e) {
			} finally {
				out.close();
			}
		}

	}

	static void debug(Object... os) {
		out.println(Arrays.deepToString(os));
	}

	private static void solve() throws IOException {
		int n = Integer.parseInt(in.readLine());
		Set<String> set = new HashSet<String>();
		for(int i =0;i<n;i++){
			String s = in.readLine();
			for(int l =0;l<s.length();l++){
				for(int r = l+1;r<=s.length();r++)
					set.add(s.substring(l,r));
			}
		}
		String check = "a";
		while(true){
			if(!set.contains(check)){
				out.println(check);
				return;
			}else{
				check = next(check);
			}
		}
	}

	private static String next(String s) {
		int n = s.length();
		int index=0;
		for(;index<n;index++)
			if(s.charAt(index)!='z'){
				break;
			}
		if(index==n){
			char[] cs = new char[n+1];
			for(int i =0;i<n+1;i++)
				cs[i]='a';
			return new String(cs);
		}
		char[] cs = s.toCharArray();
		for(int i =n-1;i>=0;i--){
			if(cs[i]!='z'){
				cs[i]=(char) (cs[i]+1);
				return new String(cs);
			}
				
		}
		
		return null;
	}

}