#include <bits/stdc++.h>
using namespace std;
#define int long long
const int INF = 1e13;
struct fenwick{
vector<int> tree;
int n;
fenwick(int n):n(n),tree(n+1){}
void add(int k,int x){
while(k<=n){
tree[k]+=x;
k+=k&-k;
}
}
int get(int k){
int ans = 0;
while(k){
ans+=tree[k];
k-=k&-k;
}
return ans;
}
};
int32_t main(){
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
int n,k;
cin >> n >> k;
string s;
cin >> s;
s.insert(s.begin(),'$');
vector<int> posA(n+1);
vector<int> posB(n+1);
int cntA = 0,cntB = 0;
for(int i=1;i<=2*n;i++){
if(s[i]=='A'){
posA[++cntA]=i;
} else {
posB[++cntB]=i;
}
}
vector<int> costA(n+1);
vector<int> costB(n+1);
vector<int> lastB(n+1);
{
fenwick temp(2*n);
for(int i=n;i;i--){
temp.add(posB[i],1);
costA[i] = temp.get(posA[i]);
}
for(int i=1;i<=n;i++)costA[i]+=costA[i-1];
}
{
fenwick temp(2*n);
for(int i=n;i;i--){
costB[i] = n-i-temp.get(posB[i]);
temp.add(posA[i],1);
}
for(int i=1;i<=n;i++)costB[i]+=costB[i-1];
}
{
int currlast = 0;
for(int i=1;i<=n;i++){
while(currlast!=n and posB[currlast+1]<posA[i])currlast++;
lastB[i] = min(currlast,i);
}
}
auto solve = [&](int penalty){
vector<pair<int,int>> DP(n+1,{INF,INF});
DP[0]={0,0};
auto cost = [&](int a,int b){
assert(a<=b);
int Ac = costA[b]-costA[a-1];
if(lastB[b]<a)return Ac+penalty;
int amt = lastB[b]-a+1;;
int Bc = costB[lastB[b]]-costB[a-1]-amt*(n-b);
return Ac+Bc+penalty;
};
deque<pair<int,int>> q;
auto test = [&](int x,int a,int b){
// Will return true if a is better for x than b
return make_pair(DP[a-1].first+cost(a,x),DP[a-1].second+1)<make_pair(DP[b-1].first+cost(b,x),DP[b-1].second+1);
};
for(int i=1;i<=n;i++){
// Add i
while(!q.empty() and q.front().second<i)q.pop_front();
while(!q.empty()){
int lower = i;
if(q.size()>1)lower = q[q.size()-2].second+1;
if(test(lower,i,q.back().first)){
q.pop_back();
continue;
}
for(int jump=524288;jump;jump/=2)if(lower+jump<=n and test(lower+jump,q.back().first,i))lower+=jump;
q.back().second = lower;
break;
}
if(q.empty() or q.back().second!=n)q.emplace_back(i,n);
// Query i
assert(!q.empty());
DP[i] = {DP[q.front().first-1].first+cost(q.front().first,i),DP[q.front().first-1].second+1};
}
return DP[n];
};
int best_penalty = -1;
for(int jump=(1ll<<40);jump;jump/=2ll){
if(solve(best_penalty+jump).second>k)best_penalty+=jump;
}
best_penalty++;
cout << solve(best_penalty).first-k*best_penalty << '\n';
}
