s = input()
mx = 0
ss = [0 for a 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)