charas = list(input())
num = 0

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':    
            for r in range(0, i+1)[::-1]:
                if charas[r+1] == 'A' and charas[r] == 'B':
                    charas[r] = 'A'
                    charas[r+1] = 'B'
                    num += 1
                else:
                    break
            break
    search(num)
    
search(num)