def bin_to_ab(decimal_num):
	a1_str=str(f"{decimal_num:b}").zfill(N).replace("0","a")
	ab_str=a1_str.replace("1","b")
	ab_str=list(ab_str)
	return ab_str

NG_words=[]
OKs=t=0

N=int(input())
for i in range(N):
	NG_words.append(list(input()))
while OKs!=N:
	for j in range(N):
		if bin_to_ab(t)==NG_words[j]:
			OKs=0
			t+=1
			break
		else:
			OKs+=1
print("".join(bin_to_ab(t)))