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 2
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 1
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 10
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import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.StringTokenizer;


public class B {	
	BufferedReader reader;
    StringTokenizer tokenizer;
    PrintWriter out;
    
	public void solve() throws IOException {				
		int N = nextInt();
		char[][] s = new char[N][];		
		boolean[][] taken = new boolean[21][26];
		for(int i = 0; i < N; i++){
			s[i] = reader.readLine().toCharArray();
			int existing_a = 0;
			for(int j = 0; j < s[i].length; j++){
				
				for(int k = 0; k <= existing_a; k++){
					taken[k][s[i][j]-'a'] = true;
				}
				
				if(s[i][j] == 'a'){
					existing_a++;
				}
				else{
					existing_a = 0;
				}
			}
		}
		
		for(int i = 0; i < taken.length; i++){
			for(int j = 0; j < 26; j++){
				if( !taken[i][j]){
					for(int k = 0; k < i; k++){
						out.print('a');
					}
					out.println( (char)(j+'a') );
					return;
				}
			}
		}
	}
	
	/**
	 * @param args
	 */
	public static void main(String[] args) {
		new B().run();
	}
	
	public void run() {
        try {
            reader = new BufferedReader(new InputStreamReader(System.in));
            tokenizer = null;
            out = new PrintWriter(System.out);
            solve();
            reader.close();
            out.close();
        } catch (Exception e) {
            e.printStackTrace();
            System.exit(1);
        }
    }

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

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

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

    String nextToken() throws IOException {
        while (tokenizer == null || !tokenizer.hasMoreTokens()) {
            tokenizer = new StringTokenizer(reader.readLine());
        }
        return tokenizer.nextToken();
    }

}
