s = input()
mx = 0
ss = [0 for j in range(len(s)+1)]

sss = {0:0}

for a , b in enumerate(s):
    if b == "A":
        ss[a+1] = ss[a] + 1
    else:
        ss[a+1] = ss[a] - 1
    if not ss[a+1] in sss:
        sss[ss[a+1]]  = a + 1

for a , b in enumerate(ss):
    mx = max(mx,a-sss[b])
print(mx)