#include <bits/stdc++.h>
using namespace std;

#define fi first
#define se second
#define pb push_back
using ll=long long;
using ull=unsigned long long;
using lll=__int128;
using db=double;
using ld=long double;
using pii=pair<int,int>;
using pll=pair<ll,ll>;
using vi=vector<int>;
using vl=vector<ll>;
using vii=vector<pii>;
using vll=vector<pll>;
mt19937 rnd(random_device{}());
mt19937_64 rnd64(random_device{}());

const int N=2e6+5;
const ll LINF=4e18;
int n,K;
char str[N];
int a[N],L[N],l[N],tot;
ll s[N];
ll f[N];
int g[N];
vi op[N];
struct point
{
	ll x,y;
	
};
point operator-(point a,point b)
{
	return {a.x-b.x,a.y-b.y};
}
ll cross(point a,point b)
{
	return a.x*b.y-a.y*b.x;
}
ll calc(point a,int i)
{
	return a.y-a.x*i+s[i];
}
pair<ll,int> check(ll W)
{
	static pair<point,int> q[N];
	int hd=1,tl=0;
	for (int i=0;i<=n+1;i++) op[i].clear();
	for (int i=0;i<=n;i++) l[i]=max(L[i],i+1);
	fill(f,f+n+1,LINF);
	f[0]=g[0]=0;
	op[l[0]].pb(0);
	int mx=0;
	for (int i=1,p=0;i<=n;i++)
	{
		while (p<i&&l[p]<=i) p++;
		if (p<i) f[i]=f[p]+W,g[i]=g[p]+1;
		for (int t:op[i])
		{
			// assert(t>=mx);mx=max(mx,t);
			pair<point,int> u={point{t,f[t]+W-s[i-1]+(i-1ll)*t},g[t]+1};
			while (hd<tl&&cross(u.fi-q[tl-1].fi,q[tl].fi-q[tl-1].fi)>=0) tl--; 
			q[++tl]=u;
		}
		while (hd<tl&&calc(q[hd].fi,i)>=calc(q[hd+1].fi,i)) 
		{
			ll v=calc(q[hd].fi,i);
			if (v<f[i]||v==f[i]&&q[hd].se<g[i]) f[i]=v,g[i]=q[hd].se;
			hd++;
		}
		if (hd<=tl)
		{
			ll v=calc(q[hd].fi,i);
			if (v<f[i]||v==f[i]&&q[hd].se<g[i]) f[i]=v,g[i]=q[hd].se;
		}
		op[l[i]].pb(i);
	}
	return {f[n],g[n]};
}
int main()
{
	scanf("%d%d",&n,&K);
	scanf(" %s",str+1);
	int cB=0;
	for (int i=1;i<=2*n;i++)
		if (str[i]=='B') cB++;
		else a[++tot]=cB;
	assert(tot==n);
	fill(L,L+n+1,n+1);
	for (int i=1;i<=n;i++) L[a[i]]=min(L[a[i]],i);
	for (int i=n-1;i>=0;i--) L[i]=min(L[i],L[i+1]);
	for (int i=1;i<=n;i++) s[i]=s[i-1]+a[i];
	ll l=0,r=1ll*n*n/2;
	while (l<r)
	{
		ll mid=l+r>>1;
		if (check(mid).se<=K) r=mid;
		else l=mid+1;
	}
	// printf("l=%lld\n",l);
	printf("%lld\n",check(l).fi-l*K);
}