S = input()
listS = list(S)
listN = [0]
listA = []
listMAX = {}
listMIN = {}
mx = 0
count = 0

for a in listS:
    if a == "A":
        count += 1
        listN.append(count)
    if a == "B":
        count -= 1
        listN.append(count)
    if listA.count(count) ==0:
        listA.append(count)
for a in listA:
    listMIN[a] = listN.index(a);

for d,e in enumerate(listN):
    listMAX[e] = d;
    
for a in listA:
    if listMAX[a] - listMIN[a] > mx:
        mx = listMAX[a] - listMIN[a]

print(str(mx) + "\n")