/* name: P9338
 * author: 5ab
 * created at: 2023-05-26
 */
#include <iostream>
#include <utility>
#include <vector>
#include <deque>
using namespace std;

typedef long long ll;
using pli = pair<ll, int>;
const int max_n = 1e6;
const ll INF = 0x3fffffffffffffff;

char s[max_n * 2 + 1];
pli dp[max_n + 1];
ll pf[max_n + 1];

inline void chmin(pli& _a, pli _b)
{
	if (_a.first > _b.first)
		_a = _b;
	else if (_a.first == _b.first && _a.second < _b.second)
		_a = _b;
}
inline void chmin(int& _a, int _b) { if (_a > _b) _a = _b; }

signed main()
{
	ios_base::sync_with_stdio(false);
	cin.tie(nullptr);
	
	int n, k;
	
	cin >> n >> k >> s;
	for (int i = 0, j = -1; i < n; i++)
	{
		for (j++; s[j] == 'A'; j++);
		pf[i + 1] = pf[i] + j - i;
	}
	
	auto expd = [&](int y, int x)
	{
		ll df = dp[x].first - dp[y].first + pf[x] - pf[y], extk = df / (x - y);
		// cerr << "? " << y << " " << x << " " << extk + (df - extk * (x - y) == 0 && dp[y].second > dp[x].second) << endl;
		return extk - (df - extk * (x - y) == 0 && dp[y].second < dp[x].second);
	};
	auto calc = [&](int i, int j, int pr)
	{
		// cerr << i << " " << pr << " " << j << " " << (pf[pr] - pf[i]) << endl;
		chmin(pr, j);
		return make_pair(dp[i].first + 1ll * j * (pr - i) - (pf[pr] - pf[i]), dp[i].second);
	};
	
	deque<int> q, stk;
	auto check = [&](ll lim)
	{
		q.clear(), stk.clear();
		
		q.push_back(0);
		auto inc = [&](pli vl) {
			return make_pair(vl.first + lim, vl.second + 1);
		};
		for (int i = 1, pret = -1; i <= n; i++)
		{
			for (pret++; s[pret] != 'A'; pret++);
			while (!q.empty() && q.front() < pret + 1 - i)
			{
				while (stk.size() > 1 && expd(stk.end()[-2], stk.back()) >= expd(stk.back(), q.front()))
				{
					// cerr << "stk pop1 " << stk.back() << endl;
					stk.pop_back();
				}
				// cerr << "stk push " << q.front() << endl;
				stk.push_back(q.front());
				q.pop_front();
			}
			while (stk.size() > 1 && expd(stk.front(), stk[1]) < i)
			{
				// cerr << "stk pop2 " << stk.front() << endl;
				stk.pop_front();
			}
			
			dp[i] = make_pair(INF, -1);
			if (!stk.empty())
				chmin(dp[i], inc(calc(stk.front(), i, pret + 1 - i)));
			if (!q.empty())
				chmin(dp[i], inc(dp[q.front()]));
			q.push_back(i);
			
			// cerr << i << " " << dp[i].first << " " << dp[i].second << endl;
		}
		return dp[n];
	};
	
	ll l = 0, r = 1ll * n * n, ans = 0;
	while (l < r)
	{
		ll mid = (l + r) >> 1;
		auto [vl, cnt] = check(mid);
		
		// cerr << "--> " << mid << " " << vl << " " << cnt << endl;
		
		if (cnt >= k)
			ans = mid, l = mid + 1;
		else
			r = mid;
	}
	auto [res, _] = check(ans);
	
	cout << res - ans * k << endl;
	
	return 0;
}
// started coding at: 05-26 08:26:42