//{{{
#include <bits/stdc++.h>
using namespace std;
#define rep(...) repN((__VA_ARGS__,rep3,rep2,loop,~))(__VA_ARGS__)
#define loop(n) rep2(_loop_,n)
#define rep2(i,n) rep3(i,0,n)
#define rep3(i,begin,end) for(int i=(int)(begin),i##_end=(int)(end);i<i##_end;++i)
#define repN(a) repX a
#define repX(a0,a1,a2,x,...) x
typedef long long ll;
const int MOD = 1e9+7;
//}}}

int main(){
	string s;
	cin >> s;
	int ans = 0;
	rep(i, s.size()){
		if(s[i] == 'B'){
			rep(j, i + 1, s.size()){
				if(s[j] == 'A') ans++;
			}
		}
	}
	cout << ans << endl;
	return 0;
}