s = input()
d = [0]
for i,x in enumerate(s):
    p = d[-1] + 1 if x == "A" else d[-1] - 1
    d.append(p)

ans = 0
b = {}
for i,x in enumerate(d):
    if d[i] not in b:
        b[x] = i
    else:
        ans = max(ans, i-b[x])
print(ans)