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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
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 e
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 aa
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import java.util.Scanner;

public class A167 {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int n= sc.nextInt();
		String arr[] = new String[n];
		for (int i = 0; i < n; i++) {
			arr[i]=sc.next();
		}
		String output="";

		int cnt=97;
		for (int k = 0; k < 26 && cnt<123; k++) {
			String tmp = output+(char)(cnt);
			boolean contain=false;
			for (int j = 0; j < arr.length; j++) {
				if(arr[j].contains(tmp)){
					contain=true;
					break;
				}
			}
			if(!contain){
				output += tmp;
				break;
			} else{
				if(cnt==122){
					k=-1; cnt=96;
					output += (char)(97);
				}
			}
			cnt++;
		}
		System.out.println(output);

	}
}
