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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
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 e
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 aa
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import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.StringTokenizer;
import java.util.TreeSet;


public class Main {

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

		int n = sc.nextInt();
		TreeSet<String> subs = new TreeSet<>();
		while(n-->0)
		{
			String s = sc.next();
			for(int i = 0; i < s.length(); ++i)
				for(int j = i; j < s.length(); ++j)
					subs.add(s.substring(i, j + 1));
		}
		String ans = "abcd";
		for(char a = 'a'; a <= 'z'; ++a)
			for(char b = 'a' - 1; b <= 'z'; ++b)
				for(char c = 'a' - 1; c <= 'z'; ++c)
				{
					String cur = "" + a;
					if(b >= 'a')
						cur += b;
					if(c >= 'a')
						cur += c;
					if(subs.contains(cur))
						continue;
					if(cur.length() < ans.length() || cur.length() == ans.length() && cur.compareTo(ans) < 0)
						ans = cur;
				}
		out.println(ans);
		out.flush();
		out.close();
	}
	
	static class Scanner 
	{
		StringTokenizer st;
		BufferedReader br;

		public Scanner(InputStream s){	br = new BufferedReader(new InputStreamReader(s));}

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

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

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

		public String nextLine() throws IOException {return br.readLine();}

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

		public boolean ready() throws IOException {return br.ready();} 
	}
}
