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 2
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 1
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 10
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import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Solver solver = new Solver();
        solver.solve(); 
    }
}

class Solver {
    Scanner scanner = new Scanner(System.in);
    
    String[] str = new String[45];
    String res;
    
    int n;
    
    boolean good(String s) {
        for (int i = 0; i < n; i++)
            if (str[i].indexOf(s) >= 0) {
                return false;
            }
        return true;
    }
    
    boolean one() {
        for (char ch = 'a'; ch <= 'z'; ch++) {
            res = "" + ch;
            if (good(res))
                return true;
        }
        return false;
    }
    
    boolean two() {
        for (char ch1 = 'a'; ch1 <= 'z'; ch1++) {
            for (char ch2 = 'a'; ch2 <= 'z'; ch2++) {
                res = "" + ch1 + ch2;
                if (good(res))
                    return true;
            }
        }
        return false;
    }
    
    boolean three() {
        for (char ch1 = 'a'; ch1 <= 'z'; ch1++) {
            for (char ch2 = 'a'; ch2 <= 'z'; ch2++) {
                for (char ch3 = 'a'; ch3 <= 'z'; ch3++) {
                    res = "" + ch1 + ch2 + ch3;
                    if (good(res))
                        return true;
                }
            }
        }
        return false;
    }
    
    public void solve() {
        n = scanner.nextInt();
        for (int i = 0; i < n; i++)
            str[i] = scanner.next();
        if (one())
            System.out.println(res);
        else if (two())
            System.out.println(res);
        else if (three())
            System.out.println(res);
        else System.out.println("F**k");
    }
}
	  	 			 				   	 		 				 		