#include<bits/stdc++.h>
# define int long long
# define rep(i, j, k) for (int i = j; i <= k; ++i)
# define inf 4000000000000000000LL
using namespace std;

int n, k;
string s;
int b[1000005], dla[1000005], pre[1000005], kkp[1000005];
int dp[1000005], cnt[1000005], stk[1000005], l, r, pas[1000005];
int calc(int l, int r)
{
    if (b[l] >= r) return 0;
    return (kkp[r] - l + 1) * r - pre[kkp[r]] + pre[l - 1];
}
int ef(int x, int y)
{
    int l = y, r = n + 1;
    while (r - l > 1)
    {
        int mid = ((l + r) >> 1), tx = dp[x] + calc(x + 1, mid), ty = dp[y] + calc(y + 1, mid);
        if (tx < ty || tx == ty && cnt[x] < cnt[y]) l = mid; else r = mid;
    }
    return r;
}
int check(int x)
{
    stk[l = r = 1] = 0;
    rep(i, 1, n)
    {
        while (l < r && pas[l] <= i) ++l;
        cnt[i] = cnt[stk[l]] + 1, dp[i] = dp[stk[l]] + calc(stk[l] + 1, i) - x;
        int ttt = 0;
        while (l < r && pas[r - 1] > (ttt = ef(stk[r], i))) --r;
        stk[++r] = i; if (r - l == 1) pas[r - 1] = ef(stk[r - 1], i); else pas[r - 1] = ttt;
    }
    return cnt[n];
}

signed main()
{
//	freopen("ex_4.in", "r", stdin);
//	freopen("a.out", "w", stdout);
	ios::sync_with_stdio(false); cin.tie(0), cout.tie(0);
	cin >> n >> k >> s;
	int nowa = 0, nowb = 0;
	rep(i, 1, 2 * n) if (s[i - 1] == 'A') dla[nowa++] = nowb; else b[++nowb] = nowa, pre[nowb] = pre[nowb - 1] + b[nowb];
	rep(i, 1, n) kkp[i] = min(i, dla[i - 1]);
//	rep(i, 0, n) cout << dla[i] << ' ' << b[i] << endl;
	int l = -2e12, r = 1e12;
	while (r - l > 1)
    {
        int mid = (l + r) / 2;
  //      cout << l << ' ' << r << ' ' << mid << endl;
        if (check(mid) <= k) l = mid; else r = mid;
    }
    check(l);
  //  cout << l << endl;
 //   rep(i, 1, n) cout << dp[i] << ' '; cout << endl;
  //  rep(i, 1, n) cout << cnt[i] << ' '; cout << endl;
    cout << dp[n] + k * l << '\n';
    return 0;
}