#include<bits/stdc++.h>

using namespace std;

#ifdef DEBUG
auto&operator<<(auto&o,pair<auto,auto>p){return o<<"("<<p.first<<", "<<p.second<<")";}
auto operator<<(auto&o,auto x)->decltype(x.end(),o){o<<"{";int i=0;for(auto e:x)o<<","+!i++<<e;return o<<"}";}
#define debug(X...)cerr<<"["#X"]: ",[](auto...$){((cerr<<$<<"; "),...)<<endl;}(X)
#else
#define debug(...){}
#endif

#define int long long
#define ll long long
#define pb push_back
#define st first
#define nd second
#define sz(x) (int)(x).size()
#define all(x) (x).begin(),(x).end()
#define FOR(i,l,r) for(int i=(l);i<=(r);i++)
#define ROF(i,r,l) for(int i=(r);i>=(l);i--)
#define endl '\n'
int inf=1000000007;
ll infl=1000000000000000007;
ll mod=998244353;

const int N=1000007;

__int128 DP[N];
int ile[N];

//int DP[N][N];
int nx[N];

__int128 pref[2*N];
__int128 P[N];

bool cmp(__int128 l1,__int128 m1,__int128 l2,__int128 m2)
{
	return l1*m2<l2*m1;
}

signed main()
{
	ios_base::sync_with_stdio(0);
	cin.tie(0);
	cout.tie(0);
	int n,k;
	cin>>n>>k;
	string s;
	cin>>s;
	vector<int>A={0},B={0};
	FOR(i,0,2*n-1)
	{
		pref[i+1]=pref[i]+(s[i]=='B');
		if(s[i]=='A')
		{
			P[sz(A)]=P[sz(A)-1]+pref[i+1];
			A.pb(i+1);
		}
		else B.pb(i+1);
	}
	int it=n+1;
	ROF(i,n,1)
	{
		while(it>i&&B[i]<A[it-1]) it--;
		nx[i]=it;
	}
	
	__int128 L=0,R=1000000000000000000,sr,ans=-1;
	while(L<=R)
	{
		sr=(L+R)/2;
		FOR(i,1,n) DP[i]=(__int128)infl*infl;
		deque<pair<__int128,__int128>>Q,F;
		it=0;
		FOR(i,0,n)
		{
			while(sz(Q)>0&&Q[0].st<=i) Q.pop_front();
			if(sz(Q)>0)
			{
				DP[i]=DP[Q[0].nd]+sr;
				ile[i]=ile[Q[0].nd]+1;
			}
			Q.pb({nx[i+1],i});
			while(it<n&&nx[it+1]<=i)
			{
				__int128 b=1000*(-P[nx[it+1]-1]+(nx[it+1]-1)*it)+DP[it];
				__int128 a=-it*1000;
				while(sz(F)>1)
				{
					int ss=sz(F);
					if(cmp(F[ss-2].nd-F[ss-1].nd,F[ss-1].st-F[ss-2].st,F[ss-1].nd-b,a-F[ss-1].st)) break;
					else F.pop_back();
				}
				F.pb({a,b});
				it++;
			}
			while(sz(F)>1)
			{
				if(F[0].st*i+F[0].nd>=F[1].st*i+F[1].nd)
				{
					__int128 X=1000*P[i]+F[0].st*i+F[0].nd+sr;
					if(X<DP[i]||(X==DP[i]&&ile[-F[0].st/1000]+1<ile[i]))
					{
						DP[i]=X;
						ile[i]=ile[-F[0].st/1000]+1;
					}
					F.pop_front();
				}
				else break;
			}
			if(sz(F)>0)
			{
				__int128 X=1000*P[i]+F[0].st*i+F[0].nd+sr;
				if(X<DP[i]||(X==DP[i]&&ile[-F[0].st/1000]+1<ile[i]))
				{
					DP[i]=X;
					ile[i]=ile[-F[0].st/1000]+1;
				}
			}
			debug(i,DP[i],ile[i]);
		}
		debug(sr,DP[n],ile[n]);
		if(ile[n]>k) L=sr+1;
		else
		{
			ans=(DP[n]-k*sr)/1000;
			debug(DP[n]-k*sr);
			R=sr-1;
		}
	}
	/*FOR(i,1,n) FOR(j,0,n) DP[i][j]=inf;
	DP[0][0]=0;
	FOR(j,1,k)
	{
		deque<pair<int,int>>Q,F;
		it=0;
		FOR(i,0,n)
		{
			
		//	if(DP[i][j]!=inf) debug(i,j,DP[i][j]);
			
			
			
			if(DP[i][j]!=inf) debug(i,j,DP[i][j]);
		}
	}
	cout<<DP[n][k]<<endl;*/
	cout<<(ll)ans<<endl;
	return 0;
}