lst = list(input())

max_length = 0
count_list = {0:-1}
count = 0

for i in range(len(lst)):
    if lst[i]=="A":
        count += 1
    else:
        count -= 1
    count_list.setdefault(count,i)
    max_length = max(max_length,i-count_list[count])

print(max_length if max_length > 1 else -1)