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)
b = sorted(set(b))
m = len(b)
if m == 1:
    if b[0] == 0:
        print('a'*n)
    else:
        print('b'*n)
    exit()

for i in range(m-1):
    if b[i]+1 != b[i+1]:
        print('a'*(b[i]+1) + 'b'*(n-b[i]-1))
        exit()
print('a'*n)