s = input()

s_list = list(s)
num_a = s_list.count('A')
border = num_a
count = 0

while 'B' in s_list[ : border] and 'A' in s_list[border : ]:
    index_b = s_list[ : border].index('B')
    s_list[index_b] = 'X'
    count += (border - 1 - index_b + 1)
    
    s_list[border - 1] = s_list[border]
    s_list[border] = 'B'
    if s_list[border - 1] == s_list[border]:
        count -= 1
    if 'B' not in s_list[ : border] or 'A' not in s_list[border : ]:
        break
    
    index_a = s_list[border : ].index('A')
    s_list[border + index_a] = 'X'
    count += (index_a + 1)
    
    s_list[border] = s_list[border - 1]
    s_list[border - 1] = 'A'
    if s_list[border - 1] == s_list[border]:
        count -= 1
        
    
print(count)