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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
b

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 a
 e
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 aa
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 a

import java.util.*;

public class A {
	
	public static void main(String args[]){
		Scanner scanner =new Scanner(System.in);
		int n=scanner.nextInt();
		int j;
		String a[]=new String[n];
		for(int i=0;i<n;i++)
			a[i]=scanner.next();
		StringBuilder sb=new StringBuilder();
		for(int i=0;i<26;i++){
			sb=new StringBuilder();
			sb.append((char)('a'+i));
			for(j=0;j<n;j++)
				if(a[j].contains(sb.toString()))
					break;
			if(j>=n){
				System.out.println(sb.toString());
				return;
			}
		}
		for(int i=0;i<26;i++){
			for(int idx=0;idx<26;idx++){
				sb=new StringBuilder();
				sb.append((char)('a'+i));
				sb.append((char)('a'+idx));
				for(j=0;j<n;j++)
					if(a[j].contains(sb.toString()))
							break;
				if(j>=n){
					System.out.println(sb.toString());
					return;
				}
			}
		}
		
	}
}
