S = input()
mem = [0]
a = b = 0
for c in S:
    if c == 'A':
        mem.append(mem[-1] + 1)
        a += 1
    else:
        mem.append(mem[-1] - 1)
        b += 1

def judge(n):
    for a,b in zip(mem, mem[n*2:]):
        if a == b:
            return True
    for a,b in zip(mem[1:], mem[n*2+1:]):
        if a == b:
            return True
    return False

for n in range(min(a,b),-1,-1):
    if judge(n):
        print(n*2)
        exit()