#include<iostream>
#include<vector>
#include<algorithm>
#include<set>
#include<unordered_map>
#include<cmath>
using namespace std;
const int MAX_N=5e3+3;
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;
void add(line l)
{
while(dp.size()>=2 && l.intersect(dp.back())<=l.intersect(dp[dp.size()-2]))
{
dp.pop_back();
bor.pop_back();
}
if(dp.size())
{
bor.back()=floor(l.intersect(dp.back()));
}
dp.push_back(l);
bor.push_back(1000000000);
}
int dpp[MAX_N][MAX_N];
int p[MAX_N];
int pp[MAX_N];
int n,k;
string s;
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];
}
for(int i=0;i<=n;i++)
{
for(int gr=0;gr<=n;gr++)
{
dpp[i][gr]=1e9;
}
}
dpp[0][0]=0;
for(int gr=1;gr<=n;gr++)
{
multiset<int>active;
unordered_map<int,vector<pair<int,int>>>here;
vector<pair<int,int>>tocheck;
dp.clear();bor.clear();
line l;
for(int i=0;i<=n;i++)
{
sort(here[i].begin(),here[i].end());
for(auto [j,id]:here[i])
{
active.erase(active.find(dpp[j][gr-1]));
l.m=-j;
l.c=-pp[id-1]+id*j-j+dpp[j][gr-1];
add(l);
}
if(active.size())dpp[i][gr]=min(dpp[i][gr],*active.begin());
int x=i;
int l=0,r=dp.size()-1;
while(l<=r)
{
int mid=(l+r)/2;
if(x<=bor[mid])
{
dpp[i][gr]=min((long long)dpp[i][gr],pp[i]+dp[mid].calc(x));
r=mid-1;
}
else l=mid+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][gr-1]!=1e9)
{
here[id].push_back({i,id});
}
active.insert(dpp[i][gr-1]);
}
}
int ans=1e9;
for(int gr=1;gr<=k;gr++)
{
ans=min(ans,dpp[n][gr]);
}
cout<<ans<<"\n";
return 0;
}
