def main1(s):
    s=[-1 if x=="A" else 1 for x in s]
    n=len(s)
    ss=[0]
    for x in s:ss.append(ss[-1]+x)
    d={}
    for i in range(n+1):
        if ss[i] in d:
            d[ss[i]].append(i)
        else:
            d[ss[i]]=[i]
    ans=0
    for v in d.values():
        if len(v)>=2:
            ans=max(ans,v[-1]-v[0])
    return ans
if __name__=='__main__':
    s=input()
    print(main1(s))