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 1
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 30
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 5
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 2
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 5
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 2
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 5
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 1
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 10
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 5
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 2
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 1
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 30
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 30
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 1
b

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 e
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 aa
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import java.util.Scanner;

public class B{
    public static String str[];
    
    public static void main(String args[]){
        Scanner input = new Scanner(System.in);
        int n = input.nextInt();
        str = new String[n];
        for(int i = 0; i < n; i++)
            str[i] = input.next();
        String ans = "";
        boolean go = true;
        for(int i = 0; i < 26; i++){
            go = true;
            ans = ((char) (i + 'a')) + "";
            for(int j = 0; j < str.length; j++){
                for(int k = 0; k < str[j].length(); k++){
                    if(str[j].substring(k, k + 1).equals(ans)){
                        go = false;
                        break;
                    }
                }
                if(!go)
                    break;
            }
            if(go)
                break;
        }
        if(go){
            System.out.println(ans);
        } else{
            go = true;
            for(int e = 0; e < 26; e++){
                String d = ((char) (e + 'a')) + "";
                for(int i = 0; i < 26; i++){
                    go = true;
                    ans = d + ((char) (i + 'a'));
                    for(int j = 0; j < str.length; j++){
                        for(int k = 0; k < str[j].length() - 1; k++){
                            if(str[j].substring(k, k + 2).equals(ans)){
                                go = false;
                                break;
                            }
                        }
                        if(!go)
                            break;
                    }
                    if(go)
                        break;
                }
                if(go){
                    System.out.println(ans);
                    break;
                }
                
            }
        }
    }
}