charas = list(input())
num = 0
if len(charas) == 1:
    print(num)
    
def search(num):
    for i in range(len(charas)):
        if i == len(charas) - 1:
            print(num)
            return 
        if charas[i] == 'B' and charas[i+1] == 'A':
            charas[i] = 'A'
            charas[i+1] = 'B'
            num += 1
            break
    search(num)
    
search(num)