#include <bits/stdc++.h>
using namespace std;
#define int long long
const int maxn = 1e6 + 5, INF = 1e18;
vector<pair<int,int>> vec;
bool sel[maxn];
int sz, mrge;
pair<int,int> solve(int mn, int mx) {
pair<int,int> dp[sz+1];
for (int i=0;i<=sz;i++) dp[i] = {INF, INF};
dp[0] = {0, 0};
for (int i=0;i<sz;i++) {
int cur = 0, val = 0;
for (int j=i;j<sz;j++) {
val += cur * vec[j].first;
if (j != i) val -= vec[j].second;
cur += vec[j].first;
if (val > mx || val < mn) continue;
dp[j+1] = min(make_pair(dp[i].first + 1, dp[i].second + val), dp[j+1]);
// cout << i << " " << j << " " << cur << " " << val << endl;
}
// cout << i << " " << dp[i].first << " " << dp[i].second << endl;
}
return dp[sz];
}
signed main() {
ios::sync_with_stdio(0); cin.tie(0);
int n, k;
cin >> n >> k;
int cnt[n+2];
for (int i=0;i<=n+1;i++) cnt[i] = 0;
int cur = 1;
for (int i=1;i<=2*n;i++) {
char x;
cin >> x;
if (x == 'B') cur++;
else cnt[cur]++;
}
int init = 0, cum = 0;
for (int i=1;i<=n;i++) {
cum += cnt[i];
if (cum < i) {
int need = i - cum;
cnt[i+1] -= need, cnt[i] += need, cum += need;
init += need;
}
}
int u = 1;
vec.push_back({cnt[1], 0});
while (u < n) {
int v = u + vec.back().first;
vec.push_back({0, 0});
for (; u < v; u++) {
vec.back().first += cnt[u+1];
vec.back().second += cnt[u+1] * (v - (u + 1));
}
}
sz = vec.size(), mrge = sz - k;
// cout << sz << endl;
// for (auto [x, y]:vec) cout << x << " " << y << endl;
int ans = 0;
int l = 0, r = 1e18;
while (l < r) {
int mid = (l+r)/2;
if (solve(0, mid).first <= k) r = mid;
else l = mid+1;
}
int MX = l;
l = 0, r = MX;
while (l < r) {
int mid = (l+r+1)/2;
if (solve(mid, MX).first <= k) l = mid;
else r = mid-1;
}
// cout << l << endl;
cout << solve(l, MX).second + init << "\n";
}
