inp=list(input())
n=len(inp)
for i in range(n):
    inp[i]=inp[i]=="A"
sl=min(sum(inp),n-sum(inp))*2
s=0
while True:
    st=inp[s:s+sl]
    if sum(st)==(sl/2):
        break
    s+=1
    if (s+sl)>n:
        sl-=2
        s=0
print(sl)