line = list(input())
l_dic = {}

for i in line:
    if i in l_dic:
        l_dic[i] +=1
    else:
        l_dic[i] =1

a = l_dic["A"]
b = l_dic["B"]

if a ==0 or b ==0:
    print(0)
elif a > b:
    print(b)
else:
    print(a)