S = input()

_max = 0
i = 0
while i < len(S):
    if S[i] != 'a':
        i += 1
        continue
    else:
        count = 1
        j = i + 1
        while j < len(S):
            if S[j] == 'a':
                count += 1
                j += 1
            else:
                break
        if count > _max:_max = count
        i = j + 1
for _ in range(_max + 1):
    print('a',end='')
print()