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import java.util.*;

public class Main{
    
    private void doit(){
        Scanner sc = new Scanner(System.in);
        while(sc.hasNext()){
            int n = sc.nextInt();
            char [][] data = new char[n][];
            for(int i = 0; i < n; i++){
                data[i] = sc.next().toCharArray();
            }
            boolean flg = false;
            for(int i = 0; i < 26; i++){
                char a = (char)(i + 'a');
                if(isOK(a + "", data)){
                    System.out.println(a);
                    flg = true;
                    break;
                }
            }

            if(! flg){
                for(int i = 0; i < 26; i++){
                    for(int j = 0; j < 26; j++){
                        char a = (char)(i + 'a');
                        char b = (char)(j + 'a');
                        String s = "" + a + b;
                        if(isOK(s, data)){
                            System.out.println(s);
                            flg = true;
                            break;
                        }
                    }
                    if(flg) break;
                }
            }

            if(! flg){
                for(int i = 0; i < 26; i++){
                    for(int j = 0; j < 26; j++){
                        for(int k = 0; k < 26; k++){
                            char a = (char)(i + 'a');
                            char b = (char)(j + 'a');
                            char c = (char)(k + 'a');
                            String s = "" + a + b + c;
                            if(isOK(s, data)){
                                System.out.println(s);
                                flg = true;
                                break;
                            }
                        }
                        if(flg) break;
                    }
                    if(flg) break;
                }
            }
        }
    }

    private boolean isOK(String string, char[][] data) {
        for(int i = 0; i < data.length; i++){
            for(int j = 0; j < data[i].length - string.length() + 1; j++){
                boolean flg = false;
                for(int k = 0; k < string.length();k++){
                    if(data[i][j + k] == string.charAt(k)){
                    }
                    else{
                        flg = true;
                    }
                }
                if(! flg) return false;
            }
        }
        return true;
    }

    private void debug(Object... o) {
        System.out.println("debug = " + Arrays.deepToString(o));
    }

    public static void main(String[] args) {
        new Main().doit();
    }
}

