#include <cstdio>
#include <string>

std::string str;

int main(){
    scanf("%s", str.c_str());

    int ans = 0, i = 0;
    while(i < sizeof(str)/sizeof(str[0])){
        if(str[i] == 'B' && str[i + 1] == 'A'){
            char tmp = str[i];
            str[i] = str[i + 1];
            str[i + 1] = tmp;

            i = 0;
            ans++;

            continue;
        }

        i++;
    }

    printf("%d\n", ans);

    return 0;
}