s = input()

t = [0] * (len(s))

for i in range(len(s)):
    if s[i] == "A":
        t[i] = 1
    else:
        t[i] = -1

dp = [0] * len(s)
for i in range(len(s)):
    dp[i] = dp[i-1] + t[i-1]

if max(dp) <= 1:
    print(0)
else:
    print(max(dp))