s = raw_input()
s = list(s)

ct = 0
flag = 1

while flag != 0:
    flag = 0
    for i in range(len(s) - 1):
        if s[i] == 'B' and s[i + 1] == 'A':
            s[i] = 'A'
            s[i + 1] = 'B'
            ct += 1
            flag = 1
print ct