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import java.util.*;

import static java.lang.System.*;

public class B278 {
    Scanner sc = new Scanner(in);
    public static Random rand=new Random();

    public String toStr(int i){
        StringBuilder str=new StringBuilder();
        while(i!=0){
            int t=i%27;
            if(t!=0)str.append((char)('a'+(i%27)-1));
            i/=27;
        }
        return str.reverse().toString();
    }

    public void run() {

        int n=sc.nextInt();
        String[] str=new String[n];
        for(int c=0;c<n;c++){
            str[c]=sc.next();
        }

        check:for(int i=0;i<Integer.MAX_VALUE;i++){
            String s=toStr(i);
            for(int c=0;c<n;c++){
                if(str[c].contains(s)){
                    continue check;
                }
            }
            ln(s);return;
        }

    }

    public static void main(String[] _) {
        new B278().run();
    }


    public int[] nextIntArray(int n){
        int[] res=new int[n];
        for(int i=0;i<n;i++){
            res[i]=sc.nextInt();
        }
        return res;
    }
    public static void pr(Object o) {
        out.print(o);
    }
    public static void ln(Object o) {
        out.println(o);
    }
    public static void ln() {
        out.println();
    }
}