S = input()
cumsum = [None]*(len(S)+1)
cumsum[0] = 1
for i in range(1,len(S)+1):
    if S[i-1] == 'A':
        cumsum[i] = cumsum[i-1] + 1
    else:
        cumsum[i] = cumsum[i-1] - 1

begin_point = {}
for i,s in enumerate(cumsum):
    if s not in begin_point:
        begin_point[s] = [i,-1]
    else:
        begin_point[s][1] = i

ans = 0
for val in begin_point.values():
    ans = max(ans,val[1]-val[0])
print(ans)