#include<iostream>
#include<stack>
#include<map>
#include<vector>
#include<string>
#include<cassert>
#include<unordered_map>
#include <queue>
#include <cstdint>
#include<cstring>
#include<limits.h>
#include<cmath>
#include<set>
#include<algorithm>
#include <iomanip>
#include<numeric>
#include<bitset>
using namespace std;
#define ll long long
#define f first
#define s second
#define pii pair<int,int>
#define ppii pair<int,pii>
#define vi vector<int>
#define pb push_back
#define all(x) x.begin(),x.end()
#define rall(x) x.rbegin(),x.rend()
#define F(n) for(int i=0;i<n;i++)
#define lb lower_bound
#define ub upper_bound
#define fastio ios::sync_with_stdio(false);cin.tie(NULL);
#pragma GCC optimize ("03,unroll-lopps")
#define int long long
#define double long double
using namespace std;
const int mod=1e9+7,mxn=2e6+5,inf=1e18,minf=-1e18,lg=30;
//#undef int
int n,k,m,x,q;
void setIO(string name){
ios_base::sync_with_stdio(0); cin.tie(0);
freopen((name+".in").c_str(),"r",stdin);
freopen((name+".out").c_str(),"w",stdout);
}
vector<int>A,B;
int prefB[mxn+10];
int pref[mxn+10],cntA[mxn+10];
int getcost(int l,int r){
l--,r--;
return (pref[A[r]]-pref[max(A[l],B[l])-1])+(cntA[A[r]]-cntA[max(A[l],B[l])-1])*(-prefB[B[l]-1]);
}
struct line{
int m,c;
int get(int x){return m*x+c;}
};
double intersect(line a,line b){return (a.c-b.c)*1.0/(b.m-a.m);}
struct cht{
deque<pair<line,int>>dq;
void add(line x,int y){
while(dq.size()>1&&(intersect(x,dq[dq.size()-2].f)<=intersect(dq[dq.size()-2].f,dq.back().f)))dq.pop_back();
dq.push_back({x,y});
}
//slope -> -prefB[B[l]-1] = -prefB[B[j+1]-1] = decreasing
//qry for min qry is also increasing
pii qry(int x){
while(dq.size()>1&&dq[0].f.get(x)>dq[1].f.get(x))dq.pop_front();
if(dq.empty())assert(0);
return {dq[0].f.get(x),dq[0].s};
}
void init(){while(!dq.empty())dq.pop_back();}
}t;
pii dp[mxn+10];
int check(int lambda){
dp[0]={0,0};
t.init();
for(int i=1;i<=n;i++)dp[i]={inf,inf};
t.add((line){0,0},1);
for(int i=1;i<=n;i++){
dp[i]=t.qry(cntA[A[i-1]]);
dp[i].f+=pref[A[i-1]]+lambda;
if(i<n){
line x={-prefB[B[i]-1],-pref[max(A[i],B[i])-1]+(cntA[max(A[i],B[i])-1]*prefB[B[i]-1])+dp[i].f};
t.add(x,dp[i].s+1);
}
}
return dp[n].s;
}
int32_t main(){
fastio
cin>>n>>k;
string a;cin>>a;
int c=0;
for(int i=1;i<=2*n;i++){
if(a[i-1]=='A')A.pb(i),cntA[i]++;
else B.pb(i),c++;
prefB[i]=c;
if(a[i-1]=='A')pref[i]=prefB[i];
pref[i]+=pref[i-1],cntA[i]+=cntA[i-1];
}
int l=0,r=1e9,ans=inf;
while(l<=r){
int mid=l+(r-l)/2;
//check -> left most point (min)
if(check(mid)<=k)r=mid-1,ans=min(ans,mid);
else l=mid+1;
}
check(ans);
cout<<dp[n].f-(ans*k);
/*
L=j,
dp[i][k] = dp[j][k-1] + (pref[AR]-pref[BL]) - (cntA[AR]-cntA[BL])*(prefB[BL-1]);
dp[j][k-1] + sum of (cnt of Bj Ai where j<i) in range (first B , last A)
- (cnt of A in range [firsB,last A])*(cnt of Bj<BL)
*/
}
