# coding: utf-8
# Your code here!

s = input()
s_size = len(s)

def count(k):
    if k==s_size:
        print(0)
        return
    
    start = 0
    end = s_size-k
    for b in range(k+1):
        count_a = s[start:end].count("A")
        if count_a == s_size-k-count_a:
            print(count_a*2)
            return
        start+=1
        end+=1
    count(k+1)

count(0)