#include <bits/stdc++.h>
using namespace std;
#ifdef LOCAL
#include "debug.h"
#else
#define debug(...) 42
#endif
#define int long long
#define fi first
#define se second
#define L(i, j, k) for (int i = (j); i <= (k); i++)
#define R(i, j, k) for (int i = (j); i >= (k); i--)
#define all(x) x.begin(), x.end()
const int inf = 1e18;
struct Line {
int m, c, id;
int eval(int x) { return m * x + c; }
long double intersectX(Line l) { return (long double) (c - l.c) / (l.m - m); }
};
void solve() {
int n, K, ca = 0, cb = 0;
string s;
cin >> n >> K >> s;
vector<int> b(n+1), pb(n+1, 0);
for (int i = 0; i < 2 * n; i++) {
if (s[i] == 'B') cb++;
else b[++ca] = cb;
}
for (int i = 1; i <= n; i++) pb[i] = pb[i - 1] + b[i];
// for (int i = 1; i <= n; i++) cout << b[i] << " \n"[i == n];
vector<int> nxt(n+1, 0);
for (int i = 0; i <= n; i++) {
nxt[i] = lower_bound(b.begin() + 1, b.begin() + n + 1, i) - b.begin();
nxt[i] = min(nxt[i], n + 1);
// cout << nxt[i] << " ";
}
// cout << '\n';
vector<vector<int>> psh(n+2);
for (int i = 0; i <= n; i++) psh[max(i + 1, nxt[i])].push_back(i);
// for (int i = 0; i <= n; i++) cout << max(i + 1, nxt[i]) << " " << pb[i] << '\n';
auto solve = [&] (int lamb) {
// cout << lamb << " " << n << '\n';
vector<Line> Q(n+2);
int L = 1, R = 0;
vector<int> dp(n+1, inf), cnt(n+1, 0);
dp[0] = 0, cnt[0] = 0;
// cout << "yo\n";
auto C = [&] (int index) {
return dp[index] - pb[max(index + 1, nxt[index]) - 1] + max(index + 1, nxt[index]) * index - index;
};
// Q[L] = {0, C(0), 0};
for (int i = 1; i <= n; i++) {
// cout << lamb << " " << i << '\n';
for (auto& x : psh[i]) {
// cout << "!! ins " << x << '\n';
Line cur = {-x, C(x), x};
while (R - L >= 1 && Q[R-1].intersectX(Q[R]) >= Q[R].intersectX(cur)) R--;
Q[++R] = cur;
}
while (R - L >= 1 && Q[L].eval(i) >= Q[L+1].eval(i)) L++;
if (L <= R) {
// for (int j = L; j <= R; j++) cout << "line " << Q[j].m << " " << Q[j].c << '\n';
dp[i] = Q[L].eval(i) + pb[i] - lamb, cnt[i] = cnt[Q[L].id] + 1;
}
}
// for (int i = 1; i <= n; i++) cout << dp[i] << " " << cnt[i] << '\n';
return pair<int, int>{dp[n], cnt[n]};
};
int l = -1e11, r = 1;
while (l < r) {
int mid = l + (r - l + 1) / 2;
pair<int, int> res = solve(mid);
if (res.second <= K) l = mid;
else r = mid - 1;
}
// int l = -1e12, r = 1;
// while (l + 1 < r) {
// int mid = (l + r) / 2;
// pair<int, int> res = solve(mid);
// if (res.se <= K) l = mid;
// else r = mid;
// }
// cout << l << " " << solve(l).second << " " << solve(l).second << '\n';
cout << solve(l).first + l * K << '\n';
}
int32_t main() {
ios::sync_with_stdio(0); cin.tie(0);
int test = 1;
// cin >> test;
while (test--) solve();
}
