n = int(input())
f = [input() for i in range(n)]
g = set()

for i in range(n):
	v = 0
	for j in range(n):
		v <<= 1
		if f[i][j] == 'b':
			v ^= 1
	g.add(v)

ar = 0
for i in range(1 << n):
	if i not in g:
		ar = i
		break

s = []
for i in range(n):
	if ar & 1:
		s.append('b')
	else:
		s.append('a')
	ar >>= 1

print("".join(s[::-1]))