#include<bits/stdc++.h>
#define endl        '\n'
#define F           first
#define S           second
#define pb          push_back
#define yes         cout<<"YES\n"
#define no          cout<<"NO\n"
#define all(x)      x.begin(),x.end()
#define allr(x)     x.rbegin(),x.rend()
#define error1(x)   cerr<<#x<<" = "<<(x)<<endl
#define error2(a,b) cerr<<"("<<#a<<", "<<#b<<") = ("<<(a)<<", "<<(b)<<")\n";
#define coutall(v)  for(auto &it: v) cout << it << " "; cout << endl;
using namespace std;
using ll = long long;
using ld = long double;

void solve() {
    ll n, k, ans = 0;
    string s1;
    cin >> n >> k >> s1;
    k = min(k, (ll)count(all(s1), '?'));

    auto calAns = [&](string &s) -> ll {
        ll cntB = count(all(s), 'B'), tot = 0;
        for(int i = 0; i < n; i++) {
            if(s[i] == 'A') tot += cntB;
            else if(s[i] == 'B') --cntB;
        }
        return tot;
    };

    auto get = [&] (string s, int mid) -> ll {
        int ka = mid, kb = k - mid;
        for(int i = 0; i < n && ka > 0; i++) {
            if(s[i] == '?') {
                s[i] = 'A';
                --ka;
            }
        }
        for(int i = n - 1; i >= 0 && kb > 0; i--) {
            if(s[i] == '?') {
                s[i] = 'B';
                --kb;
            }
        }
        return calAns(s);
    };
    ans = get(s1, 0); // Initial ans

    int lo = 0, hi = k, mid;
    while(lo <= hi) {
        mid = lo + hi >> 1;
        ll cur = get(s1, mid);
        if(ans <= cur) {
            ans = cur;
            lo = mid + 1;
        }
        else hi = mid - 1;
    }

    cout << ans << endl;
    return;
}

signed main() {
    ios::sync_with_stdio(false); cin.tie(0);
    int tc = 1;
    cin >> tc;
    for (int t = 1; t <= tc; t++) {
        // cout << "Case " << t << ": ";
        solve();
    }
    return 0;
}