3
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 1
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 30
b
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 5
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 2
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 5
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abcdefghijklmnopqrst
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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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avptgpjmenfzfxwckmt
 30
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 1
b

a
 a
 e
 a
 a
 y
 b
 aa
 a
 ab
 z
 e
 b
 aa
 l
 a

import math;
import io;
import random;
import itertools as its;

INF = 9876543219876543219;

def read_int() :
	return map(int, raw_input().split());

def read_float() :
	return map(float, raw_input().split());

def read_str() :
	return raw_input().split();

def ispal(strin) :
		ii = 0;
		jj = len(strin)-1;
		while ii < jj :
			if strin[ii] != strin[jj] :
				return False;
				pass
			ii += 1;
			jj -= 1;
			pass
		
		return True;

def charint(x) :
	return ord(x) - ord('a');

if __name__ == '__main__':
	
	total = input();
	
	data = ''.join([raw_input()+" " for _ in xrange(total)]);
	alpha = [chr(ii) for ii in xrange(ord('a'), ord('z')+1)];
	
	con = True;
	idx = 1;
	while con :
		for ii in its.combinations_with_replacement(alpha, idx) : 
			now = ''.join(ii);
			#print now;
			if now not in data :
				print now;
				con = False;
				break;

		idx += 1;

	pass
