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import java.util.Map;
import java.io.IOException;
import java.io.OutputStreamWriter;
import java.io.BufferedWriter;
import java.util.InputMismatchException;
import java.util.HashMap;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.io.Writer;
import java.io.InputStream;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 * @author Diego Huerfano ( diego.link )
 */
public class Main {
	public static void main(String[] args) {
		InputStream inputStream = System.in;
		OutputStream outputStream = System.out;
		InputReader in = new InputReader(inputStream);
		OutputWriter out = new OutputWriter(outputStream);
		TaskB solver = new TaskB();
		solver.solve(1, in, out);
		out.close();
	}
}

class TaskB {

    private class Trie {
        private class Node {
            Node[] children;
            int id;
            Node( ) {
                children=new Node[26];
                id=cnt++;
            }
        }
        private Node root;
        private int cnt;
        private Map<Integer, Integer> memo;

        Trie() {
            cnt=0;
            root=new Node();
            memo=new HashMap<Integer, Integer>();
        }
        public void addWord( String word ) {
            addWord( word, 0, root );
        }
        private void addWord( String word, int ini, Node node ) {
            if( ini==word.length() ) return;
            int key=word.charAt( ini )-'a';
            if( node.children[key]==null )
                node.children[key]=new Node();
            addWord( word, ini+1, node.children[key] );
        }

        public String getAnswer( ) {
            return getAnswer( root );
        }

        private String getAnswer( Node node ) {
            for( int i=0; i<26; i++ ) {
                if( node.children[i]==null )
                    return String.valueOf((char) (i + 'a'));
            }
            int min=Integer.MAX_VALUE;
            int keymin=-1;
            for( int i=0; i<26; i++ ) {
                int aux=getMinLength( node.children[i] );
                if( 1+aux<min ) {
                    min=1+aux;
                    keymin=i;
                }
            }
            return (char)(keymin+'a') + getAnswer( node.children[keymin] );
        }

        private int getMinLength( Node node ) {
            if( memo.containsKey( node.id ) )
                return memo.get( node.id );
            for( int i=0; i<26; i++ ) {
                if( node.children[i]==null )
                    return 1;
            }
            int min=Integer.MAX_VALUE;
            for( int i=0; i<26; i++ ) {
                min=Math.min( min, 1+getMinLength( node.children[i] ) );
            }
            memo.put( node.id, min );
            return min;
        }

    }

    public void solve(int testNumber, InputReader in, OutputWriter out) {
        final int n=in.readInt();
        Trie trie=new Trie();
        for( int i=0; i<n; i++ ) {
            String s=in.readString();
            for( int j=0; j<s.length(); j++ )
                trie.addWord( s.substring( j ) );
        }
        out.printLine( trie.getAnswer() );
    }
}

class InputReader {

    private InputStream stream;
    private byte[] buf = new byte[1024];
    private int curChar;
    private int numChars;
    private SpaceCharFilter filter;

    public InputReader(InputStream stream) {
        this.stream = stream;
    }

    public int read() {
        if (numChars == -1)
            throw new InputMismatchException();
        if (curChar >= numChars) {
            curChar = 0;
            try {
                numChars = stream.read(buf);
            } catch (IOException e) {
                throw new InputMismatchException();
            }
            if (numChars <= 0)
                return -1;
        }
        return buf[curChar++];
    }

    public String readString() {
        int c = read();
        while (isSpaceChar(c))
            c = read();
        StringBuilder res = new StringBuilder();
        do {
            res.appendCodePoint(c);
            c = read();
        } while (!isSpaceChar(c));
        return res.toString();
    }

    public int readInt() {
        int c = read();
        while (isSpaceChar(c))
            c = read();
        int sgn = 1;
        if (c == '-') {
            sgn = -1;
            c = read();
        }
        int res = 0;
        do {
            if (c < '0' || c > '9')
                throw new InputMismatchException();
            res *= 10;
            res += c - '0';
            c = read();
        } while (!isSpaceChar(c));
        return res * sgn;
    }

    public boolean isSpaceChar(int c) {
        if (filter != null)
            return filter.isSpaceChar(c);
        return isWhitespace(c);
    }

    public static boolean isWhitespace(int c) {
        return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
    }

    public interface SpaceCharFilter {
        public boolean isSpaceChar(int ch);
    }
}

class OutputWriter {
    private final PrintWriter writer;

    public OutputWriter(OutputStream outputStream) {
        writer = new PrintWriter(new BufferedWriter(new OutputStreamWriter(outputStream)));
    }

    public OutputWriter(Writer writer) {
        this.writer = new PrintWriter(writer);
    }

    public void print(Object...objects) {
        for (int i = 0; i < objects.length; i++) {
            if (i != 0)
                writer.print(' ');
            writer.print(objects[i]);
        }
    }

    public void printLine(Object...objects) {
        print( objects );
        writer.println();
    }

    public void close() {
        writer.close();
    }
}

