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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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 2
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 1
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 30
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 1
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 aa
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import java.util.Scanner;


public class B {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		Unique u = new Unique();
		u.solve();
		u.print();
	}

}

class Unique {
	
	Unique() {
		Scanner scr = new Scanner(System.in);
		n = scr.nextInt();
		a = new char[n][];
		alphabet = new int[26];
		
		String s;
		char[] st;
		for (int i = 0; i < n; i++){
			s = scr.next();
			st = s.toCharArray();
			a[i] = new char[st.length];
			
			for (int j = 0; j < st.length; j++){
				a[i][j] = st[j];
				alphabet[(int)st[j] - (int)'a']++;
			}
			
		}		
	}
	
	boolean one() {
		
		for (int k = 0; k < 26; k++){
			if (alphabet[k] == 0){
				ans = "" + (char)(k + (int)'a');
				return true;
			}
		}
		return false;
	}
	
	boolean two() {
		int count;
		for (char z = 'a'; z <= 'z'; z++){
			for (char y = 'a'; y <= 'z'; y++){
				count = 0;
				for (int i = 0; i < n; i++){
					for (int j = 0; j < (a[i].length-1); j++){
						if ((z == a[i][j]) && (y == a[i][j+1])) {
							count++;
						}
					}
				}
				if (count == 0) {
					ans = "" +  z + y;
					return true;
				}	
			}			
		}
		return false;
	}
	
	boolean three() {
		int count;
		for (char z = 'a'; z <= 'z'; z++){
			for (char y = 'a'; y <= 'z'; y++){
				for (char x = 'a'; x <= 'z'; x++) {
					count = 0;
					for (int i = 0; i < n; i++){
						for (int j = 0; j < (a[i].length-2); j++){
							if ((z == a[i][j]) && (y == a[i][j+1]) && (x == a[i][j+2])) {
								count++;
							}
						}
					}
					if (count == 0) {
						ans = "" +  z + y;
						return true;
					}	
				}	
			}			
		}
		return false;
	}

	
	void solve() {
		if (one()) return;
		if (two()) return;
		if (three()) return;
		ans = "aaaa";		
	}
	
	void print(){
		System.out.println(ans);
	}
	
	String ans;
	int[] alphabet;
	char[][] a;
	int n;
}