#include <bits/stdc++.h>
using namespace std;
using ll = long long; using pii = pair<ll,ll>;
const ll Nm = 1e6+5; const ll INF = 1e9;
ll c[Nm];
ll psc[Nm+1];
ll io[Nm]; //first i with c[i0]-l>=0
ll N;
pii fz(pii p1, pii p2) {
if (p1.first<p2.first) {
return p1;
}
if (p1.first>p2.first) {
return p2;
}
if (p1.second<p2.second) {
return p1;
} else {
return p2;
}
}
vector<array<ll,3>> chull;
pii qryH(ll x) {
pii pf = {INF,INF};
for (auto A0: chull) {
pf = fz(pf,{A0[0]+A0[1]*x,A0[2]});
}
return pf;
}
pii qry(ll Ctest) {
chull.clear();
pii dp[N+1]; //{cost,# used}
for (ll i=1;i<=N;i++) {
dp[i]={INF,INF};
}
dp[0]={0,0};
deque<pii> d0;
vector<array<ll,3>> lns[N+1];
for (ll l=0;l<=N;l++) {
//process
// cout << "\nprocess l="<<l<<"\n";
// cout << "i0="<<io[l]<<"\n";
for (auto A0: lns[l]) {
chull.push_back(A0);
}
dp[l]=fz(dp[l],qryH(l-1));
//cout << "dp[l]="<<dp[l].first<<","<<dp[l].second<<"\n";
if (dp[l].first==INF) {
continue;
}
if (io[l]==N) {
dp[N]=fz(dp[N],{dp[l].first+Ctest,dp[l].second+1});
} else {
array<ll,3> A0 = {Ctest+dp[l].first+psc[l]-psc[io[l]]+l*(io[l]-1),-l,dp[l].second+1};
//cout << "A0: "<<A0[0]<<","<<A0[1]<<","<<A0[2]<<"\n";
//lns[io[l]].push_back({Ctest+dp[l].first+psc[l]-psc[io[l]]+l*(io[l]-1),-l,dp[l].second+1});
if (io[l]==l) {
chull.push_back(A0);
} else {
lns[io[l]].push_back(A0);
}
}
}
return {dp[N].first-dp[N].second*Ctest,dp[N].second};
}
int main() {
ios_base::sync_with_stdio(false); cin.tie(0);
ll K; cin >> N >> K;
string s; cin >> s;
ll i0 = 0; ll c0 = 0;
psc[0]=0;
for (ll i=0;i<(2*N);i++) {
if (s[i]=='A') {
c[i0]=c0; psc[i0+1]=psc[i0]+c[i0];
//cout << "i0="<<i0<<", c[i0]="<<c[i0]<<"\n";
i0++;
} else {
assert(s[i]=='B');
c0++;
}
}
ll iov = N;
io[N]=N;
for (ll l=(N-1);l>=0;l--) {
while (iov>0 && (c[iov-1]>=l)) {
iov--;
}
io[l]=max(l,iov);
}
// for (ll q=0;q<=25;q++) {
// cout << "q="<<q<<"\n";
// pii p0 = qry(q);
// cout << "query value="<<p0.first<<","<<p0.second<<"\n";
// }
//exit(0);
ll Cmin = 0; ll Cmax = INF/2; //aliens constant
//binary search for the smallest C where you use
//at most K blocks
while (Cmin<Cmax) {
ll Ctest = (Cmin+Cmax)/2;
pii p0 = qry(Ctest); //{cost, # of blocks}
if (p0.second<=K) {
Cmax = Ctest;
} else {
Cmin = Ctest+1;
}
}
pii p0 = qry(Cmin);
assert(p0.second<=K);
pii p1 = qry(Cmin-1); //lower cost, higher # of blocks
if (p0.second==K || p0.second==p1.second) {
cout << (p0.first+psc[N])<<"\n";
} else {
ll fv = p0.first-(K-p0.second)*(p0.first-p1.first)/(p1.second-p0.second);
cout << (fv+psc[N]) <<"\n";
}
}
