#include<iostream>
#include<vector>
#include<algorithm>
#include<set>
#include<unordered_map>
#include<cmath>
using namespace std;
const int MAX_N=1e5+6;
const long long INF=(1LL<<60);
int p[MAX_N];
int pp[MAX_N];
int n,k;
string s;
struct line
{
long double m,c;
long long calc(long long x){return m*x+c;}
long double intersect(line l)
{
return (long double)(c-l.c)/(l.m-m);
}
};
vector<line>dp;
vector<int>bor,mincnt;
void add(line l,int cnt)
{
while(dp.size()>=2 && l.intersect(dp.back())<=l.intersect(dp[dp.size()-2]))
{
dp.pop_back();
bor.pop_back();
mincnt.pop_back();
}
if(dp.size())
{
bor.back()=floor(l.intersect(dp.back()));
}
dp.push_back(l);
bor.push_back(1000000000);
mincnt.push_back(cnt);
}
multiset<pair<long long,int>>active;
vector<pair<int,int>>here[MAX_N];
long long dpp[MAX_N];
int ints[MAX_N];
int lastgroups;
long long lastdp;
line l;
int groups(long long penalty)
{
active.clear();
for(int i=0;i<=n;i++)here[i].clear();
dp.clear();bor.clear();mincnt.clear();
for(int i=1;i<=n;i++){dpp[i]=INF;}
dpp[0]=0;
ints[0]=0;
for(int i=0;i<=n;i++)
{
for(auto&[j,id]:here[i])
{
active.erase(active.find({dpp[j],ints[j]}));
l.m=-j;
l.c=-pp[id-1]+id*j-j+dpp[j];
add(l,ints[j]);
}
if(active.size())
{
long long top;
int curints;
tie(top,curints)=*active.begin();
if(top<dpp[i])
{
dpp[i]=top;
ints[i]=curints+1;
}
else if(top==dpp[i])
{
ints[i]=min(ints[i],curints+1);
}
}
int x=i;
int l=0,r=dp.size()-1;
int mid;
long long top;
int curints;
int pos;
while(l<=r)
{
int mid=(l+r)/2;
if(x<=bor[mid])
{
pos=mid;
top=dp[mid].calc(x)+pp[i];
curints=mincnt[mid];
if(top<dpp[i])
{
dpp[i]=top;
ints[i]=curints+1;
}
else if(top==dpp[i])
{
ints[i]=min(ints[i],curints+1);
}
r=mid-1;
}
else l=mid+1;
}
for(int mid=max(0,pos-20);mid<min(pos+20,(int)dp.size());mid++)
{
top=dp[mid].calc(x)+pp[i];
curints=mincnt[mid];
if(top<dpp[i])
{
dpp[i]=top;
ints[i]=curints+1;
}
else if(top==dpp[i])
{
ints[i]=min(ints[i],curints+1);
}
}
l=i+1;r=n;
int id=-1;
while(l<=r)
{
int mid=(l+r)/2;
if(p[mid]>i)
{
id=mid;
r=mid-1;
}
else l=mid+1;
}
if(id!=-1 && dpp[i]!=INF)
{
here[id].push_back({i,id});
}
if(dpp[i]!=INF){if(i){dpp[i]+=penalty;}active.insert({dpp[i],ints[i]});}
lastdp=dpp[i];
lastgroups=ints[i];
}
return lastgroups;
}
int main ()
{
ios_base::sync_with_stdio(0);
cin.tie(NULL);
cout.tie(NULL);
cin>>n>>k;
cin>>s;
int pos=0;
int cnt=0;
for(int i=0;i<2*n;i++)
{
if(s[i]=='A')
{
pos++;
p[pos]=cnt;
}
else cnt++;
}
for(int i=1;i<=n;i++)
{
pp[i]=pp[i-1]+p[i];
}
long long l=0,r=+(1LL<<40);
long long ans=-1;
while(l<=r)
{
long long mid=(l+r)/2;
if(groups(mid)<=k)
{
ans=lastdp-k*mid;
r=mid-1;
}
else l=mid+1;
}
cout<<ans<<"\n";
return 0;
}
