#include<iostream>
#include<vector>
#include<algorithm>
#include<set>
#include<unordered_map>
using namespace std;
const int MAX_N=5e3+3;
int dp[MAX_N][MAX_N];
int cost[MAX_N][MAX_N];
int p[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 pr=0;pr<=n;pr++)
{
for(int i=1;i<=n;i++)
{
cost[i][pr]=max(0,p[i]-pr)+cost[i-1][pr];
}
}
for(int i=0;i<=n;i++)
{
for(int gr=0;gr<=n;gr++)
{
dp[i][gr]=1e9;
}
}
dp[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;
for(int i=0;i<=n;i++)
{
for(auto [j,id]:here[i])
{
active.erase(active.find(dp[j][gr-1]));
tocheck.push_back({j,id});
}
if(active.size())dp[i][gr]=min(dp[i][gr],*active.begin());
for(auto [j,id]:tocheck)
{
dp[i][gr]=min(dp[i][gr],dp[j][gr-1]+p[i]-p[id]+(i-id+1)*j);
}
int 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)here[id].push_back({i,id});
active.insert(dp[i][gr-1]);
}
}
int ans=1e9;
for(int gr=1;gr<=k;gr++)
{
ans=min(ans,dp[n][gr]);
}
cout<<ans<<"\n";
return 0;
}
