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import java.util.Scanner;
import java.util.TreeSet;

public class Main
{
	public static void main( String... args ) 
	{
		Scanner scanner = new Scanner( System.in );
		int n = Integer.parseInt( scanner.nextLine() );
		String[] array = new String[n];
		for( int i = 0; i < n; i++ )
			array[i] = scanner.nextLine();
		int count = 0;
		TreeSet<Character> tree = new TreeSet<>();
		for( int i = 0; i < n; i++ )
			for( int j = 0; j < array[i].length(); j++ )
			{
				if( j == 0 )
					count = array[i].length();
				else
					if( array[i].length() < count )
						count = array[i].length();
				if( !tree.contains( array[i].charAt( j ) ) )
					tree.add( array[i].charAt( j ) );
			}
		char i;
		for( i = 97; i < 123; i++ )
			if( !tree.contains( i ) )
				break;
		
		if( i >= 122 )
		{
			i = 97;
			System.out.print( i );
			char aux = ++i;
			System.out.print( aux );
			System.out.println();
		}
		else
			System.out.println( i );
	}
}