n = int(input())
a = [input() for _ in range(n)]
b = []
for i in range(n):
    o = 0
    for j in range(n):
        if a[i][j] == 'a':
            o += 1
    b.append(o)
for i in range(n+1):
    if i not in b:
        print('a'*i + 'b'*(n-i))
        exit()