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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 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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abcdefghijklmnopqrst
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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
b

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

n=int(raw_input());
str1=[]
for i in range(0,n):
	str1.append(raw_input());
i=97;c=97;ctr=0;ctr1=0;
while ctr<len(str1):
	for s in str1[ctr]:
		if str(s)==chr(i):	
			i=i+1;
	ctr=ctr+1;
if i<=97 and i<=122:
	print chr(i);
else:
	i=97;ctr=0;
	while i<=122:
		ctr=0;
		while ctr<len(str1):
			c1=c;
			for i1 in range(len(str1[ctr])-1):
				temp=str(str1[ctr]);
				if temp[i1:i1+2]==(chr(i)+chr(c)):	
					c=c+1;
			ctr=ctr+1;
		if(c1==c):
			break;
		if(c==122):
			c=97;
			i=i+1;	
	print (chr(i)+chr(c));

