s = input()
a = [0]*len(s)
a[0] = 1 if s[0] == "A" else -1
table = {0: [0, 0], a[0]: [1, 0]}
for i in range(1, len(s)):
    a[i] = a[i-1] + (-1, 1)[s[i]=="A"]
    if a[i] not in table:
        table[a[i]] = [i+1, 0]
    else:
        table[a[i]][1] = i+1

result = [b-a for a, b in table.values()]
print(max(result))