s = str(input())
n = len(s)
C = [0]*(n+1)
for i in range(n):
    if s[i] == 'A':
        C[i+1] = C[i]+1
    else:
        C[i+1] = C[i]-1
#print(C)

d = {}
ans = 0
for i, c in enumerate(C):
    if c in d:
        ans = max(ans, i-d[c])
    else:
        d[c] = i
print(ans)