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import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.IOException;
import java.util.Arrays;
import java.util.PriorityQueue;
import java.util.ArrayList;
import java.io.OutputStreamWriter;
import java.io.BufferedWriter;
import java.util.InputMismatchException;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.io.Writer;
import java.math.BigInteger;
import java.io.InputStream;
import java.util.PriorityQueue;
import java.util.HashMap;
import java.util.Stack;
import java.io.BufferedInputStream;
import java.io.BufferedOutputStream;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.PrintStream;
import java.lang.reflect.Field;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;
import java.util.TreeSet;
public class Main{
    public static void main(String []args)throws IOException{
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        InputReader in = new InputReader(inputStream);
        //BufferedReader x=new BufferedReader(new InputStreamReader(System.in));
        OutputWriter outt = new OutputWriter(outputStream);
        StringBuilder out=new StringBuilder();
        int n=in.readInt();
        String a[]=new String[n];
        for(int i=0;i<n;i++){a[i]=in.readString();}
        HashMap<String ,Integer> z=new HashMap<String,Integer>();
        for(int i=0;i<n;i++){
        int l=a[i].length();
        for(int j=0;j<l;j++){
        z.put(String.valueOf(a[i].charAt(j)),0);
        }
        for(int j=0;j<l-1;j++){
        z.put(a[i].substring(j,j+2),0);
        }
        for(int j=0;j<l-2;j++){
        z.put(a[i].substring(j,j+3),0);
        }
        }
        char sm='a';boolean found=true;
        while(sm<='z'){boolean f=false;
        if(!z.containsKey(String.valueOf(sm))){found=false;break;}
        sm++;
        }
        if(!found){outt.printLine(sm);}
        else{
            char sm1='a';
            char sm2='a';
            while(sm1<='z'){
            while(sm2<='z'){
            String t=String.valueOf(sm1)+sm2;
            if(!z.containsKey(t)){found=false;break;}
            sm2++;
            }
            if(!found){break;}
            sm1++;
            }
            if(!found){outt.printLine(String.valueOf(sm1)+sm2);}
            else{
                char sm3='a';char sm4='a';char sm5='a';
                while(sm3<='z'){
                while(sm4<='z'){
                while(sm5<='z'){
                String t1=String.valueOf(sm3)+sm4+sm5;
                if(!z.containsKey(t1)){found=false;break;}
            sm5++;
                }
                if(!found){break;}
            sm4++;
                }
                if(!found){break;}
            sm3++;
                }
                if(!found){outt.printLine(String.valueOf(sm3)+sm4+sm5);}
            }
        }
        //outt.printLine(Math.min(sum1,sum2));
        outt.close();
        
        
    }
   
}

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 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 String readString() {
        int c = read();
        while (isSpaceChar(c))
            c = read();
        StringBuffer res = new StringBuffer();
        do {
            res.appendCodePoint(c);
            c = read();
        } while (!isSpaceChar(c));
        return res.toString();
    }
 
    public boolean isSpaceChar(int c) {
        if (filter != null)
            return filter.isSpaceChar(c);
        return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
    }
 
    public String next() {
        return readString();
    }
 
    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();
    }}