#include <iostream>
#include <algorithm>
#include <deque>
#include <vector>
#include <array>
#define ll long long
using namespace std;
ll n, k, dp[2][1000000];
vector <ll> A, B, nxA, ps;
string S;
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;
}
void solve(ll l, ll r, ll ql, ll qr) {
ll mid = (l+r)/2;
ll opt = 1e18, optid = -1, tmp;
for (int i=ql; i<mid; ++i) {
tmp = dp[0][i] + C(i+1, mid);
if (opt > tmp) {
opt = tmp;
optid = i;
}
}
dp[1][mid] = opt;
if (l < mid) solve(l, mid-1, ql, optid);
if (mid < r) solve(mid+1, r, optid, qr);
}
int main() {
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());
}
}
for (int i=0; i<n; ++i) dp[0][i] = C(0, i);
for (int x=1; x<k; ++x) {
solve(0, n-1, 0, n-1);
swap(dp[0], dp[1]);
}
cout << dp[0][n-1] << '\n';
}
