#include <iostream>
#include <algorithm>
#include <deque>
#include <vector>
#include <array>
#include <cmath>
#define ll long long
#define ld long double
using namespace std;
ll n, k, l, r, mid, dp[1000000];
ll tot[1000000];
ld ep = 1e-8;
vector <ll> A, B, nxA, ps;
string S, T;
struct Line{
ll m;
ll c;
ll x;
};
ld intx(Line I, Line J) {
return (ld)(I.c-J.c)/(J.m-I.m);
}
ll C(ll l, ll r) {
if (nxA[l] > r) return 0;
ll nx = max(l, nxA[l]), cnt = r-nx+1, cnt2 = max(0LL, l-nxA[l]);
return ps[r]-(nx ? ps[nx-1] : 0)-cnt*B[l]-cnt*(cnt-1)/2-(r-l+1)*cnt2;
}
ll f(ll i) {
ll nx = max(i, nxA[i]), cnt2 = max(0LL, i-nxA[i]);
return -(nx ? ps[nx-1] : 0)+nx*B[i]-B[i] + (-nx*nx+nx)/2 + cnt2*i - cnt2;
}
ll g(ll j) {
return ps[j] + (-j*j-j)/2;
}
ll h(ll i) {
return nxA[i]-B[i];
}
void solve() {
deque <Line> dq;
dq.push_back({h(0), f(0), 0});
for (ll i=0; i<n; ++i) {
while (dq.size() > 1) {
auto I = dq.front();
dq.pop_front();
auto J = dq.front();
if ((ld)i-intx(I, J) > ep || fabs((ld)i-intx(I, J)) <= ep) continue;
else {
dq.push_front(I);
break;
}
}
auto X = dq.front();
dp[i] = X.m * i + X.c + g(i) + mid;
tot[i] = X.x+1;
if (i != n-1) {
Line K = {h(i+1), dp[i] + f(i+1), tot[i]};
while (dq.size() > 1) {
auto J = dq.back();
dq.pop_back();
auto I = dq.back();
if (intx(I, J)-intx(I, K) > ep || fabs(intx(I, J)-intx(I, K)) <= ep) continue;
else {
dq.push_back(J);
break;
}
}
dq.push_back(K);
}
}
}
int main() {
cin.tie(0);
ios::sync_with_stdio(0);
cin >> n >> k >> S;
for (int i=0; i<2*n; ++i) {
if (S[i] == 'A') {
ps.push_back((ps.empty() ? 0 : ps.back()) + i);
A.push_back(i);
}
else {
B.push_back(i);
nxA.push_back(A.size());
}
}
l = 0, r = n*n;
while (l < r) {
mid = (l+r+1)/2;
solve();
if (k > tot[n-1]) r = mid-1;
else l = mid;
}
mid = l;
solve();
cout << dp[n-1] - k * mid << '\n';
}
