S = input()

cumsum = [0]

for i,s in enumerate(S):
    if s == "A":
        cumsum.append(cumsum[i]+1)
    else:
        cumsum.append(cumsum[i]-1)
        
#print(cumsum)

d = {}
for i, v in enumerate(cumsum):
    if v not in d:
        d[v] = [i,0]
    else:
        d[v][1] = i 
        
ans = 0
for v in d.values():
    ans = max(ans, v[1]- v[0])
    
print(ans)