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

public class B278 {
	
	static public void check(int n, String[] str, StringBuilder newString) {
		for(int i=0;i<n;i++)
			if(str[i].contains(newString))
				return;
		System.out.println(newString);
		System.exit(0);
	}
	
	static public void main(String args[]) throws Exception{
		Scanner reader = new Scanner(System.in);
		
		int n = reader.nextInt();
		String str[] = new String[n];
		for(int i=0;i<n;i++)
			str[i] = reader.next();
		StringBuilder ns = null;
		for(int i=0;i<26;i++) {
			ns = new StringBuilder();
			ns.append((char)('a'+i));
			check(n, str, ns);
		}
		for(int i=0;i<26;i++) {
			ns = new StringBuilder();
			ns.append((char)('a'+i));
			for(int k=0;k<26;k++) {
				ns.append((char)('a'+k));
				check(n, str, ns);
				ns.deleteCharAt(1);
			}
		}
		for(int i=0;i<26;i++) {
			ns = new StringBuilder();
			ns.append((char)('a'+i));
			for(int k=0;k<26;k++) {
				ns.append((char)('a'+k));
				for(int z=0;z<26;z++) {
					ns.append((char)('a'+z));
					check(n, str, ns);
					ns.deleteCharAt(2);
				}
				ns.deleteCharAt(1);
			}
		}
	}
}
