S = input()
dictN = {0:0}
listA = []
dictMIN = {}
mx = 0
count = 0

for a,b in enumerate(S):
    if b == "A":
        count += 1
    else:
        count -= 1
    if count in dictMIN:
        dictN[count] = a
    else:
        dictMIN[count] = a
        
    if not count in listA:
        listA.append(count)
for a in listA:
    try:
        if dictN[a] - dictMIN[a] > mx:
            mx = dictN[a] - dictMIN[a]
    except KeyError:
        pass

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