#include <cstdio>
#include <string>


#define size(s) sizeof(s)/sizeof(s[0])
std::string str;

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

    int ans = 0, cnt = 0, f = 0;
    for(;;){
        f = 1;
        for(int i = 1; i < size(str); i++){
            if(str[i - 1] == 'B' && str[i] == 'A'){
                char tmp = str[i - 1];
                str[i - 1] = str[i];
                str[i] = tmp;

                ans++;
                f = 0;
            }
        }

        if(f) break;
    }

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

    return 0;
}