#include <bits/stdc++.h>
#define int long long
using namespace std;
#define all(v) (v).begin(), (v).end()
#define rall(v) (v).rbegin(), (v).rend()
#define rep(i, a, b) for(int i = (a); i < (b); i++)
#define sz(v) ((int)((v).size()))
template<typename T>
void chmax(T &x, const T &v) { if (x < v) x = v; }
template<typename T>
void chmin(T &x, const T &v) { if (x > v) x = v; }
using pii = pair<int, int>;
using vi = vector<int>;
string to_string(string s) { return s; }
template <typename T> string to_string(T v) {
bool first = true;
string res = "[";
for (const auto &x : v) {
if (!first)
res += ", ";
first = false;
res += to_string(x);
}
res += "]";
return res;
}
void dbg_out() { cout << endl; }
template <typename Head, typename... Tail> void dbg_out(Head H, Tail... T) {
cout << ' ' << to_string(H);
dbg_out(T...);
}
#ifdef DEBUG
#define dbg(...) cout << "(" << #__VA_ARGS__ << "):", dbg_out(__VA_ARGS__)
#else
#define dbg(...)
#endif
int N, K;
string S;
const int INF = 1e12;
// ferme[i] = nombre d'ouvrants avant Fi
vector<int> ouvre, ferme;
vector<int> cumuFerme;
int coutInclus(int left, int right) {
if (left > right) {
return 0;
}
int projExclus = min(right+1, ouvre[right]);
if (projExclus <= left) return 0;
return (projExclus - left)*(right+1) - (cumuFerme[projExclus] - cumuFerme[left]);
}
vector<vector<int>> dp;
// cur et opt inclus
// mais cur représente right exclus
void calc(int nbCoup, int curLeft, int curRight, int optLeft, int optRight) {
if (curLeft > curRight) return;
int curMid = (curLeft + curRight) / 2;
pair<int, int> p {INF, -1};
rep(coupe, optLeft, optRight+1) {
chmin(p, {dp[nbCoup-1][coupe] + coutInclus(coupe, curMid-1), coupe});
}
dp[nbCoup][curMid] = p.first;
int optMid = p.second;
calc(nbCoup, curLeft, curMid-1, optLeft, optMid);
calc(nbCoup, curMid+1, curRight, optMid, optRight);
}
signed main() {
ios::sync_with_stdio(false);
cin.tie(0);
cin >> N >> K >> S;
{
int curOuvre = 0, curFerme = 0;
cumuFerme.push_back(0);
for (char c : S) {
if (c == 'A') {
++curOuvre;
ouvre.push_back(curFerme);
} else {
++curFerme;
ferme.push_back(curOuvre);
cumuFerme.push_back(cumuFerme.back() + curOuvre);
}
}
}
dp = vector<vector<int>>(K+1, vector<int>(N+1, INF));
dp[0][0] = 0;
rep(nbCoup, 1, K+1) {
calc(nbCoup, 0, N, 0, N-1);
}
cout << dp[K][N] << '\n';
}
