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

for a,b in enumerate(S):
    if a == "A":
        count += 1
    else:
        count -= 1
    listN[count] = b
        
    if not count in listA:
        listA.append(count)


for a in listA:
    listMIN[a] = listN.index(a);
    if listN[a] - listMIN[a] > mx:
        mx = listMAX[a] - listMIN[a]

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